rgw/cache: this commit squashes the following commits for ssd backed cache driver.

rgw/cache: this commit squashes the following commits for ssd backed cache driver.

initial implementation of ssd backed cache driver.This contains implementations
for methods related to Entry and Partition.

adding methods for put, get and delete data.
They are all synchronous.

added CacheAioRequest to handle async read and write
requests and added a corresponding read op in aio.h and aio.cc.

added get_async method implementation and
initial implementation to handle async reads using asio.

changing the signature of list_entries to match that with
base class and making it public.

adding implementation for asynchronous get.

modifying namespace from rgw::cal to rgw::cache.
Modifying the SSDDriver constructor for Partition related
modifications.

Signed-off-by: Pritha Srivastava <prsrivas@redhat.com>