Fix protobuf detection on systems with Python 3 as default (part2)

MESOS-1774

Review: https://reviews.apache.org/r/25439
Send pending tasks during re-registration.

Review: https://reviews.apache.org/r/25371
Made the GarbageCollector injectable into the Slave.

Review: https://reviews.apache.org/r/25372
Added a test for sending pending tasks during re-registration.

Review: https://reviews.apache.org/r/25373
Fix git clean -xdf skipping leveldb, removes internal .git dirs
from leveldb tarball via 'git archive --format=tar.gz
--prefix=leveldb/ origin/master > leveldb.tar.gz'

MESOS-1764

Review: https://reviews.apache.org/r/25508
Fixed command executor path check

Review: https://reviews.apache.org/r/25205