Refactor a quorum-size calculation

No behavior change, just use the utility function instead of rolling
our own.

Change-Id: I152ccd8f22f424cd7547fa4d67df020f263096a9
AUTHORS and CHANGELOG update for 1.13 release

Change-Id: I6195c70cbbd7c94682539962dd5410e3665f01ac
Merge "Refactor a quorum-size calculation"
Merge "AUTHORS and CHANGELOG update for 1.13 release"