Introduce CacheAllocator, a custom allocator for cache blocks (#4437)

Introduce CacheAllocator, a custom allocator for cache blocks (#4437)

Summary:
This is a conceptually simple change, but it touches many files to
pass the allocator through function calls.

We introduce CacheAllocator, which can be used by clients to configure
custom allocator for cache blocks. Our motivation is to hook this up
with folly's `JemallocNodumpAllocator`
(https://github.com/facebook/folly/blob/f43ce6d6866b7b994b3019df561109afae050ebc/folly/experimental/JemallocNodumpAllocator.h),
but there are many other possible use cases.

Additionally, this commit cleans up memory allocation in
`util/compression.h`, making sure that all allocations are wrapped in a
unique_ptr as soon as possible.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4437

Differential Revision: D10132814

Pulled By: yiwu-arbug

fbshipit-source-id: be1343a4b69f6048df127939fea9bbc96969f564