Convert Token Memcache backend to new KeyValueStore Impl

This patchset converts the memcache backend for tokens to the new
KeyValueStore implementation.  This implementation provides basic
configuration choices for utilizing memcache as a token backend.

The test_backend_memcache test suite has been removed, as there is
no extra functionality added on top of the KeyValueStore base
besides the basic configuration choices.  The test_backend_memcache
test suite was only used for testing token memcache backend.  The
important unicode tests have been moved into the base token test
suite and should now validate against all token backends.

Closes-Bug: #1260080
bp: dogpile-kvs-backends
Change-Id: I8f0bca409a6393176e77f7a27ba15959756da06f