Disk cache refactoring

[chaubold]

As discussed in Dresden in December 2018, we've been working on a N5 backend for the DiskCachedCellImg. To prepare for that, I have refactored the DiskCachedCellImgFactory and DiskCachedCellImgOptions to allow for better extensibility and reuse. Their external interface remains (nearly) unchanged.

The two main (internal) changes are:

• I have introduced an AbstractReadWriteCachedCellImgFactory and AbstractReadWriteCachedCellImgOptions that hold the code that can be shared by different read-write-cache implementations. Concrete subclasses must implement abstract methods that encapsulate backend-specific things like CellCache creation.
• Options (and their Values) now inherit from each other to remove code duplication, as DiskCached options were a strict superset of the ReadOnlyCached options: DiskCachedCellImgOptions extends AbstractReadWriteCachedCellImgOptions extends ReadOnlyCachedCellImgOptions

API change:

• DiskCachedCellImgOptions.values is now private, use the new method values() instead to get access to the values.
• CacheType has moved from DiskCachedCellImgOptions to ReadOnlyCachedCellImgOptions because that is its first usage in the inheritance hierarchy.

Notes:

• As there are no tests in this repository, I have run all examples from imglib2-cache-examples to verify that the caches still work as expected.
• The N5 backend that makes use of this refactoring is currently located in https://github.com/chaubold/imglib2-cache-n5. The reason is that n5-imglib provides a N5CellLoader that we reuse instead of reimplementing it, but n5-imglib at the same time depends on imglib2-cache. To prevent cyclic dependencies, I have put the N5CachedCellImg into its own maven project.
• We also talked about maybe sharing an IoSync instance across multiple caches. I did not add an interface for this yet.
• For KNIME, we actually implemented a Flushable version of the GuardedStrongRefLoaderRemoverCache to be able to trigger writing of all in-memory cache blocks to disk. This is not handled in this PR or imglib2-cache-n5 yet.

[chaubold]

CC: @gab1one @MarcelWiedenmann @dietzc @HedgehogCode