R1:e4ea092f60de

Implement a chunked, APC-backed graph cache

Summary:
Ref T2683. This is a refinement and simplification of D5257. In particular:

- D5257 only cached the commit chain, not path changes. This meant that we had to go issue an awkward query (which was slow on Facebook's install) periodically while reading the cache. This was reasonable locally but killed performance at FB scale. Instead, we can include path information in the cache. It is very rare that this is large except in Subversion, and we do not…