fix a thundering herd problem

fix a thundering herd problem

Avoid repeatedly opening keys db when accessing a local git remote and -J
is used.

What was happening was that Remote.Git.onLocal created a new annex state
as each thread started up. The way the MVar was used did not prevent that.
And that, in turn, led to repeated opening of the keys db, as well as
probably other extra work or resource use.

Also managed to get rid of Annex.remoteannexstate, and it turned out there
was an unncessary Maybe in the keysdbhandle, since the handle starts out
closed.