In-memory AC cache. [WIP]

[excitoon]

This fix adds in-memory cache for AC records.

Resolves #92

[excitoon]

What I would also like to make in this PR:

• move from AC cache to entries-smaller-than-N-bytes cache;
• add number and size limit for these entries (like existing LRU).

[mostynb]

I wonder if the kernel's page cache already performs an approximation of this optimization transparently? Have you benchmarked this compared to the filesystem backend?