Currently getting the read write concern while creating each opctx is the source of the a significant number of branch misses in a 100% findById workload. Caching this value to avoid branching in the most common case were the default does not change improves throughput ~1% or reduces cpu util ~1%

Proposed Fix

Add a cheap atomic boolean and accessor in read_write_concern_defaults.h and a fast-path early return at top of _extractReadConcern:

See this patch for proposed change. 

Impact

This multipatch shows a 1.3% improvement in our 100 read YCSB workload.