SERVER-15089 prep tryToGoLiveAsSecondary for moving into applier
SERVER-15089 convert non-legacy usage of lastOpTimeWritten to replCoord's lastOpTimeApplied
SERVER-15089 minvalid helper functions; move tryToGoLiveAsASecondary() to sync_tail.cpp
SERVER-15089 move the applier threadpools into SyncTail
SERVER-14233 remove support for old oplog versions