SERVER-13951 Split index building in to UnitOfWork-sized stages

All index builds now go through the MultiIndexBuilder as its API was already
close to ideal. The following tickets have also been addressed by this commit:

SERVER-14710 Remove dropDups
SERVER-12309 Cloner build indexes in parallel
SERVER-14737 Initial sync uses bg index building
SERVER-9135 fast index build for initial sync
SERVER-2747 can't kill index in phase 2
SERVER-8917 check error code rather than assuming all errors are dups
SERVER-14820 compact enforces unique while claiming not to
SERVER-14746 IndexRebuilder should be foreground and fail fatally
SERVER-13951 WUOW now takes whole OperationContext, not just RecoveryUnit

This will make it possible to implement two-phase locking.