Dump currently only takes the read lock. Xapian can cope with some
changes while maintaining a read snapshot but with more changes it
fails. Currently notmuch just gives a xapian error.
To avoid this we take the write lock when dumping. This prevents other
notmuch processes from modifying the xapian database preventing this
error.
Discussion with Olly on irc indicates that this is currently the best
solution: in xapian trunk there may be better possibilities using
snapshots but they need to make it to a release and propogate out to
users before we can switch approach.
Finally, this breaks one use case: pipelines of the form
notmuch dump | ... | notmuch restore
According to Olly this is already very fragile: it will only work on
small databases. One of the tests relies on this behaviour so fix that
to store the dump rather than use a pipe.
All test scripts to be executed are now named as T\d\d\d-name.sh,
numers in increments of 10.
This eases adding new tests and developers to see which are test scripts
that are executed by test suite and in which order.
