allow nodes to start as forced_slave (using quiesced mechanism)
allow configuring forced_slave from the extensions
use forced_slave in a bunch of tests
fix test_restart_single_slave_short
check if forced slave before doing quiesced operations
assert db_optimize and db_defrag throw on forced slaves
fix some tests
add test to assert forced_slave can collapse
test inject_as_head on forced slaves
don't wrap defrag call in try_quiesced as this results in an invalid operation
fix test_is_progress_possible, add test_is_progress_possible_forced_slave
Merge #175 remote-tracking branch 'refs/remotes/origin/ARAKOON-359-2' into origin-1.7

Conflicts:
	src/node/node_main.ml