Environments need to read the DB a lot when installing all specs.

- [x] Put a read transaction around `install_all()` and `install()`
  to avoid repeated locking

Our `LockTransaction` class was reading overly aggressively.  In cases
like this:

```
1  with spack.store.db.read_transaction():
2    with spack.store.db.write_transaction():
3      ...
```

The `ReadTransaction` on line 1 would read in the DB, but the
WriteTransaction on line 2 would read in the DB *again*, even though we
had a read lock the whole time.  `WriteTransaction`s were only
considering nested writes to decide when to read, but they didn't know
when we already had a read lock.

- [x] `Lock.acquire_write()` return `False` in cases where we already had
       a read lock.

If a write transaction was nested inside a read transaction, it would not
write properly on release, e.g., in a sequence like this, inside our
`LockTransaction` class:

```
1  with spack.store.db.read_transaction():
2    with spack.store.db.write_transaction():
3      ...
4  with spack.store.db.read_transaction():
   ...
```

The WriteTransaction on line 2 had no way of knowing that its
`__exit__()` call was the last *write* in the nesting, and it would skip
calling its write function.

The `__exit__()` call of the `ReadTransaction` on line 1 wouldn't know
how to write, and the file would never be written.

The DB would be correct in memory, but the `ReadTransaction` on line 4
would re-read the whole DB assuming that other processes may have
modified it.  Since the DB was never written, we got stale data.

- [x] Make `Lock.release_write()` return `True` whenever we release the
      *last write* in a nest.

Lock transactions were actually writing *after* the lock was
released. The code was looking at the result of `release_write()` before
writing, then writing based on whether the lock was released.  This is
pretty obviously wrong.

- [x] Refactor `Lock` so that a release function can be passed to the
      `Lock` and called *only* when a lock is really released.

- [x] Refactor `LockTransaction` classes to use the release function
  instead of checking the return value of `release_read()` / `release_write()`

`ViewDescriptor.regenerate()` checks repeatedly whether packages are
installed and also does a lot of DB queries.  Put a read transaction
around the whole thing to avoid repeatedly locking and unlocking the DB.

Add latest release.

* version bump

	modified:   var/spack/repos/builtin/packages/py-slepc4py/package.py

* slepc: update URL
slepc4py: add 3.11.0 and update maintainers

Co-authored-by: Satish Balay <balay@mcs.anl.gov>

* build bazel with jdk

* Flake 8

* Fix typo

Users can now list mirrors of the main url in packages.

- [x] Instead of just a single `url` attribute, users can provide a list (`urls`) in the package, and these will be tried by in order by the fetch strategy.

- [x] To handle one of the most common mirror cases, define a `GNUMirrorPackage` mixin to handle all the standard GNU mirrors.  GNU packages can set `gnu_mirror_path` to define the path within a mirror, and the mixin handles setting up all the requisite GNU mirror URLs.

- [x] update all GNU packages in `builtin` to use the `GNUMirrorPackage` mixin.

* Add symbols patch

* Apply symbols patch to pgmath

* Add github issue number for symbols patch.

* Add naromero77 as a maintainer.

* Patch only applied to March 2019 release and master.

* node-js: add Python 3 support

* Update node-js, fix Python 3 support in v12

* Record that old versions of ROOT don't support modern GCC

* Well, actually I don't know about 6.07

* Fix typo and follow odd version recommendation from @chissg

* Add QE 6.5

* Support for serial HDF5 case with serial (no mpi) QE is now supported but requires a patch for 6.4.1 and 6.5.

* Add naromero77 as a maintainer.

* Start cinema package

* Remove boilerplate and add description

* Formatting for pep8

* Correct milestone tag

* 'master' instead of 'develop'

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

* Two variants, both with numpy and other small changes

* When +image for scikit

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>