SERVER-14101 explain for update
SERVER-14977: add new alias for sslWeakCertificateValidation option
SERVER-14668 Make Locker an interface

Turns the Locker class into an interface and changes most usages of
LockState to now be either accesses to this interface or to use the
default interface implementation.
SERVER-14668 make getLockMode/isLockHeldForMode pure virtual
SERVER-14668 split dbdirectclient out of instance.cpp
SERVER-15202: Restore boost 1.49 as default
SERVER-15219 race in migration commit can cause it to timeout
SERVER-12637: log a startup warning when mongod/mongos is run as root.
Merge remote-tracking branch 'upstream/master'