mon: Monitor: add timecheck infrastructure to detect clock skews

Fixes: #3633

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
mon: Monitor: reduce indentation level; make code more readable

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: move a couple of if's together on handle_command()

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: use 'else if' on handle_command instead of bunches of 'if'

... when the options are mutually exclusive.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: unify 'ceph health' and 'ceph status'; add json output

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>