CXX-182 initial WriteConcern implementation
CXX-182 tests for WriteConcern class
CXX-182 add OperationException class
CXX-182 integrate WriteConcern into interface
CXX-182 fix existing tests that throw exceptions now
minor: 100k is probably enough, boiling oceans, etc
CXX-182 updates requested after second review
CXX-182 third review updates
CXX-182 comments and minor changes after review