Allow to schedule a task to run gc for all projects

The (global) schedule can be configured in "gerrit.config" by specifying
startTime and interval. The "startTime" can be specified in section "gc"
in the following format:
startTime = <hours>:<minutes>
or
startTime = <day of week> <hours>:<minutes>
The "interval" can be given using the usual time unit suffixes.

Change-Id: I16e92c353d0fa05c020fc18da4ed1f5825ad677c
8 files changed