Two small refactoring for better inlining

Two small refactoring for better inlining

Summary:
Move uncommon code paths in RangeDelAggregator::ShouldDelete() and IterKey::EnlargeBufferIfNeeded() to a separate function, so that the inlined strcuture can be more optimized.

Optimize it because these places show up in CPU profiling, though minimum. The performance is really hard measure. I ran db_bench with readseq benchmark against in-memory DB many times. The variation is big, but it seems to show 1% improvements.
Closes https://github.com/facebook/rocksdb/pull/2877

Differential Revision: D5828123

Pulled By: siying

fbshipit-source-id: 41a49e229f91e9f8409f85cc6f0dc70e31334e4b