Major stuff:
- Created a FileCache for managing user cache files in Spack. Currently just
handles virtuals.
- Moved virtual cache from the repository to the home directory so that users do
not need write access to Spack repositories to use them.
- Refactored `Transaction` class in `database.py` -- moved it to
`LockTransaction` in `lock.py` and made it reusable by other classes.
Other additions:
- Added tests for file cache and transactions.
- Added a few more tests for database
- Fixed bug in DB where writes could happen even if exceptions were raised
during a transaction.
- `spack uninstall` now attempts to repair the database when it discovers that a
prefix doesn't exist but a DB record does.
102ac7bcf1