Add 'DropMaster' command

See: ARAKOON-311
See: http://jira.incubaid.com/browse/ARAKOON-311
added python tests for dropMaster
updated test_drop_master
unit test for drop_master; wait a lease period after expiration before returning to client
This should do it ;)
Merge remote-tracking branch 'origin/wip-ARAKOON-311' into 1.6

Conflicts:
	src/client/client_protocol.ml
	src/tlog/mem_tlogcollection.ml