Use flock to guard against concurrent cache access

[samliddicott]

Lock the local dir with flock -Fs for shared read locks and flock -Fx when updating

This prevents certain failure at concurrency >=5 and succedes past concurrency of 70

Change-Id: I5beda9168b13b0525dc108c2ca85f9d21e126780

[jacobschloss]

I've had some trouble with the cache server seeming to corrupt already-cached repos on client fetch/update with 6 CI machines. I'll try this patch out.

[samliddicott]

I've had some trouble with the cache server seeming to corrupt already-cached repos on client fetch/update with 6 CI machines. I'll try this patch out.

Did it help @jacobschloss ?

[jacobschloss]

It helped, decreased from near constant corruption to maybe once every few weeks (6 machines on either 10G or 25G LAN).