Changed checkpoint logging from LOG(INFO) to VLOG(1).

Review: https://reviews.apache.org/r/26242
Fixed a log line in the master.

Review: https://reviews.apache.org/r/26198
Eliminated redundant resource accounting in the master.

Review: https://reviews.apache.org/r/26199
Added validation for missing ExecutorInfo::framework_id.

Review: https://reviews.apache.org/r/26200
Properly deprecated ReregisterSlaveMessage::slave_id.

Review: https://reviews.apache.org/r/26201
Introduced a version during slave (re-)registration.

Review: https://reviews.apache.org/r/26202
Removed redundant logging in the Master.

Review: https://reviews.apache.org/r/26203
Removed the need for Master::readdSlave.

Review: https://reviews.apache.org/r/26204
Cleaned up Master::addFramework.

Review: https://reviews.apache.org/r/26205
Introduced Mesos modules abstractions.

Adding a first class primitive, abstraction and process for dynamic
library writing and loading can make it easier to extend inner
workings of Mesos. Making it possible to have dynamic loadable
resource allocators, isolators, containerizes, authenticators and much
more.

Review: https://reviews.apache.org/r/25848
Added style/syntax changes made to modules abstractions.

This was done by Kapil Arya, Niklas Nielsen, and myself.