Updated ReviewBot to look at old reviews first.

Review: https://reviews.apache.org/r/20617
Add a return statement to Varint64Comparator::Compare in leveldb to
silence the compiler warning. This patch fixes this build error with
ICC.

Review: https://reviews.apache.org/r/21292
Moved ReconcileTaskTest test to its own file.

Review: https://reviews.apache.org/r/21175
Fixed tasks reconciliation and added tests.

Review: https://reviews.apache.org/r/21236
Fixed white space in authorizer.hpp
Added slave operation counters to Master.

Review: https://reviews.apache.org/r/21290
Allowed Gauge to take synchronous functions.

Review: https://reviews.apache.org/r/20995
Added --override flag to command executor.

It will be a common use case that external containerizer programs will
have to be able to control the command being carried out when the only
the task.command or container image is present.  The command executor
will in its current state try to run the command in the context of the
slave and not the container.

So, you could imagine the command executor being used by: 1) Replace
the command to run when the slave sends a runTask message 2) Prefix a
command to wrap call in when the slave sends a runTask message.

1) have been implemented and tested in this patch, alongside making
the command-executor "cli-ized" with help messages.

Review: https://reviews.apache.org/r/21329