Parse the following new syntax for readConcern:

{readConcern: {level: "snapshot"}}
{readConcern: {atClusterTime: <timestamp>}}

Parsing for snapshot:

• Add level "snapshot" to the readConcern interface.
  • Add "snapshot" level definition to the ReadConcernArgs class.
  • Parse in ReadConcernArgs::initialize().
• Update ReplicationCoordinatorImpl::_validateReadConcern() to allow the readConcern level snapshot with afterClusterTime.
• Return an error if the mongod is not a member of a replica set.
• Support readConcern level snapshot for the commands aggregate, count, distinct, group, geoNear, geoSearch, find, insert, update, delete, findAndModify.
• Ban on mongos.

Parsing for atClusterTime:

• Add readConcern parsing / initialization components (ReadConcernArgs::initialize()).
  • Remove the "testMode" check.
  • Error if the level is not snapshot.
  • Error if paired with afterOpTime or afterClusterTime.
• Update ReplicationCoordinatorImpl::_validateReadConcern() to check that the readConcern level is snapshot if atClusterTime is provided.