client: add option for minimum merge delay
client: add option for minimum merge delay

If the Log in use is known to have a lower bound on the time
before a submitted leaf will be included (for example, the read
staleness for the cloudspanner storage layer), this option can
be set to prevent unnecessary polling.

daviddrysdale/trillian,gdbelvin/trillian,Martin2112/trillian,Martin2112/trillian,AlCutter/trillian,AlCutter/trillian,Martin2112/trillian,google/trillian,daviddrysdale/trillian,daviddrysdale/trillian,RJPercival/trillian,google/trillian,gdbelvin/trillian,RJPercival/trillian,gdbelvin/trillian