Changes to add "writecache" mode - write only caching

Summary: Commit contains 2 changes
1) Add a "writecache" mode, a variant of writeback caching where
only incoming writes are cached.
2) Improves disk write clustering for disk cleaning by reading
all the dirty blocks from flash in a chain and then issuing large
write for the entire chain (flashcache_kcopy.c).

Test Plan:

Reviewers: Herman Lee (facebook).

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