In SERVER-33311, we need to be able to store the readConcernLevel in a wiredtiger recovery unit. setReadFromMajorityCommittedSnapshot() seems like the right place to set readConcernLevel since we call setReadFromMajorityCommittedSnapshot() in getMore command to set the readConcernLevel to MajorityCommitted.

setReadFromMajorityCommittedSnapshot() should probably be renamed to setReadConcernLevel() and the boolean member variable _readFromMajorityCommittedSnapshot should be extended to a ReadConcernLevel enum class called _readConcernLevel.