Abstract caching and support entrypoints

[banesullivan]

Abstracts the caching mechanism so that external projects can implement a cache interface and use entrypoints to make it available to large_image.

Generally, this is working for django-large-image with girder/django-large-image#32

Todo:

• Can the MemCache cache be chosen appropriately? Is it ignored when not available and is it chosen over python when available?
• Should loadCaches() only be called once rather than on every CacheFactory.getCache() call?
• Address inline TODO statements
• What is CacheFactory.getCacheSize()and pickAvailableCache() and how are they affected by these changes?
• What happens if BaseCache.getCache() returns a None cache lock? can/should we support that?
• Test downstream in django-large-image

[banesullivan]

@manthey, this seems to be working and is tested downstream.I'm ready for a final review if you have time

[banesullivan]

Can we also tag 1.14.6 (or should this be 1.15 since there is a new interface) after this merges?

[manthey]

Excellent.

[manthey]

What happens if BaseCache.getCache() returns a None cache lock? can/should we support that?

That should work correctly.

[manthey]

Can we also tag 1.14.6 (or should this be 1.15 since there is a new interface) after this merges?

It will be 1.15.0 since I think this is enough of a new interface.