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.
Replaced hard tabs with spaces in module/manager.cpp.

Review: https://reviews.apache.org/r/26467
Fixed framework logging in master.cpp.

Review: https://reviews.apache.org/r/26159
Introduced Master <-> Slave reconciliation.

Review: https://reviews.apache.org/r/26206
Split out partition and reconciliation tests from FaultTolerace tests.

Review: https://reviews.apache.org/r/26207
Added a test for the Master <-> Slave reconciliation race.

Review: https://reviews.apache.org/r/26208
Disallowed duplicate module names.

Module manager should return error on encountering a duplicate module
name (i.e. if a module with the same name has already been loaded).

Review: https://reviews.apache.org/r/26513
Update metrics::Timer::stop to return elapsed time.

Review: https://reviews.apache.org/r/24537
Added batch sizes and timings to the Registrar logging.

Review: https://reviews.apache.org/r/26535
Added markdown formatting to the committer's guide.