Cache manager refactor

[c-w]

@cpeel: I did a bit of refactoring on your metadata cache manager code in order to gain an understanding of it. I'd like to get some feedback on whether I understood everything correctly before I merge.

The gist of this refactor:

• Create sqlite/bsddb subclasses of MetadataCache to avoid store-type checking in methods
• Create testing MetadataCache to remove testing-only code in the api.
• Fall back to sqlite cache when the user doesn't have bsddb installed.

[cpeel]

The set_metadata_cache() function is not just for testing, but rather an integral part of my branch. Without a way to explicitly set a cache, you can't control where the cache is stored on the filesystem, or in the new code, control which of the two backend types you want to use. This level of control is vital when using this as part of a larger service.

For an example of what I'm trying to achieve with this module, see:
https://github.com/cpeel/openlibrary/blob/095df4b3be522eef218813a0106ee2ce7934f120/openlibrary/plugins/gutenberg/pgebook.py
and
https://github.com/cpeel/openlibrary/blob/73533a3e011074e2ffbfe416f241a6aa14fb5b6e/scripts/dev-instance/populate_gutenberg_cache.py

[c-w]

Fixed in 8df3e24.

[cpeel]

Overall I like your refactor -- very clean. My one big concern is the removal of set_metadata_cache[_manager]() as that function enables one of the core requirements of what I was trying to achieve -- the ability to control where the cache ends up on the filesystem and what type of cache it is.

[cpeel]

Built-in python errors (almost) all end in Exception, Error, or Warning (https://docs.python.org/2/library/exceptions.html). See also PEP8: https://www.python.org/dev/peps/pep-0008/#exception-names

(I should have raised this comment against the commit when the exceptions.py file went in, as they're all this way.)

[c-w]

Fixed in 8921ee1.

[c-w]

• Added exception suffixes in 8921ee1.
• Froze dependency on sqlalchemy-rdflib in d887023.
• Brought back set_metadata_cache in 8df3e24.

[cpeel]

Ship it! :)

[c-w]

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