reference: add wait options for local reference activation

reference: add wait options for local reference activation

Add "waitunsynced" option to specify how long chronyd needs to wait
before it can activate the local reference when the clock is not
synchronized to give the configured sources a chance to synchronize the
local clock after start. The default is 300 seconds when the orphan
option is enabled (same as the ntpd's default orphanwait), 0 otherwise.

Add "waitsynced" option to specify how long it should wait when the
clock is synchronized. It is an additional requirement to the distance
and activate options.