cr: don't count merged updates when deciding to take the lock (#687)
cr: don't count merged updates when deciding to take the lock (#687)

If this device has say one unmerged update, but there are 500+ merged
updates and they keep coming, then taking the local lock isn't going
to do any good because it doesn't stop the merged updates from coming.
Also, it might take a while to download those merged updates, and
since we only let ourselves take the lock for 10 seconds, CR will
never complete.

So this PR just doesn't count merged devices when taking a lock.  I
filed KBFS-1896 to explore doing "partial" CRs when the set of merged
updates is too large.

Issue: KBFS-1881
keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client