change preferred master implementation
nodes should not behave in a preferred manner after receiving a drop master call
make node behave 'preferred' again after a maintenance operation
cleaner start_preferred_master_loop , fix TODO related to using ssl context
Merge branch 'incubaid-1.7' into incubaid-preferred-master-1.7

Conflicts:
	src/client/client_main.ml
wait a bit longer after the defrag before checking the preferred node is the master again
sleep in test_preferred_master after drop_master call should not be needed
comments from PR
Merge pull request #480 from Incubaid/preferred-master-1.7

Preferred master 1.7