SERVER-15248 Make ScatterGatherRunner, ElectCmdRunner and FreshnessChecker cancelable.

This is necessary so that step-downs can cancel running elections.
SERVER-15215 Add method to ReplCoordExternalState to load the most recent optime from the oplog.
SERVER-15215 Load last optime from oplog when a local config is found at startup.
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
Merge remote-tracking branch 'upstream/master'