add failing drop_master_while_maintenance
make sure lease expired event keep forever flowing through the state machine
prevent slave from ignoring duplicate message from master
a bit more info logging
Merge branch 'incubaid-1.6' into masterless

Conflicts:
	src/node/mem_store.ml
	src/paxos/slave.ml
	src/system/drop_master.ml
comment from PR
Merge pull request #460 from Incubaid/masterless

Masterless