First step in implementing linearizable reads: implement read concern level.

When a read is received with a read concern level “linearizable”, we must first check that the current node is master (available for writes). Nodes that are not currently in writable state shall return a NotMaster error. Linearizable reads are not permitted on Secondary nodes.

If an afterOpTime read concern option is also supplied, a parse error is returned. To limit project scope, afterOpTime read concern option will be incompatible with read concern level linearizable, for now.

Additionally, need to include unittests for parsing.