Remove usages of System.currentTimeMillis() from gerrit-reviewdb

Unlike pure server code, reviewdb.client is used from GWT, so we
cannot just use joda's DateTimeUtils.currentTimeMillis(). Instead,
just introduce arguments where necessary. Passing TimeUtil.nowTs() is
a bit cumbersome but usually short enough to type.

Change-Id: I87ef09b0920bc36851bca893a2a37348ffc6bf3a
38 files changed