Per-account locking

[Sebastian Spaeth]

I have now pushed the per-account locks to my 'next' branch. For
backwards-compatability, the old global lock is still in place, so you
won't benefit from this goodness. After some releases (next major
number?) we can then decide to drop support for the old global lock by
simply reverting commit 0d9565141765b8b23c1c723d325cf494e47cc80d.

I know that Nicolas would have preferred some cleverer solution where we
figure out the correct lock to use ourselves. But each time I tried, I
ran into issues that required more brain than I was willing to use. So
this is the 2nd-best compromise. Introduce new-style lock now and in a
few months, we can drop the global one. (Or we could introduce a
temporary setting that disables the global lock for those who know they
won't be running an old offlineimap for sure).

Sebastian

Commit logs follow:
commit 0d9565141765b8b23c1c723d325cf494e47cc80d
Perform legacy global lock in addition to new per-account lock

We simply lock OfflineImap the same global way that we have always done
in addition to the previously implemented per-account lock. We can keep
both systems in parallel and then after a few stable releases, drop the
old-style global lock. by reverting this patch

commit 89c705bb2656b8edf028ce569329a012b4515c3e
init.py: Import OfflineImapError

The next commit will make use of OfflineImapError but is transient (the
old-style lock). The commit is supposed to be reverted after a few
releases. So add the new import in a separate commit, because we might
need this even when reverting the commit.

commit c7938dc0819e0a14d1a97e2362e46daa86a7ce85
Per-account locking

Previously, we were simply locking offlineimap whenever it was
running. Howver there is no reason why we shouldn't be able to invoke it
in parallel, e.g. to synchronize several accounts in one offlineimap
each.

This patch implements the locking per-account, so that it is possible to
sync different accounts at the same time. If in refresh mode, we will
attempt to loop three times before giving up.

This also fixes Debian bug #586655
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