Cache refreshed loose ref dirs in SnapshottingRefDirectory

Cache refreshed loose ref dirs in SnapshottingRefDirectory

Update SnapshottingRefDirectory to have a cache of dirs refreshed for
loose refs. This should help improve performance when 'after_open'
setting is used for 'trustLooseRefStat' as duplicate refreshes are
avoided when a snapshot of the ref database is used in a request scope.

Change-Id: I8f66e7cee572e477d29abe2d9db69e97bca3ee4c
Signed-off-by: Nasser Grainawi <quic_nasserg@quicinc.com>
Co-authored-by: Martin Fick <quic_mfick@quicinc.com>
Co-authored-by: Kaushik Lingarkar <quic_kaushikl@quicinc.com>