CXX-206 Backport server 2.6.0..2.7.0 changes

Files merged manually or fixed up:
 SConstruct
 src/mongo/bson/bsonobj.h
 src/mongo/bson/optime.cpp
 src/mongo/bson/optime.h
 src/mongo/client/connpool.h
 src/mongo/client/dbclientinterface.h
 src/mongo/db/repl/member_state.h
 src/mongo/dbtests/mock/mock_replica_set.cpp
 src/mongo/util/assert_util.h
 src/mongo/util/log.h
 src/mongo/util/time_support.h
CXX-227 Implement hygenic build support
CXX-227 Remove more obsolete items from .gitignore
CXX-227 Clarify comment per review feedback
CXX-227 With hygenic builds, no longer need --prefix in travis
CXX-226 Error out if we fail to find boost headers at configure time
minor: add MacOSX system path for libc++ to ycm_extra_conf
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