Revise the locking API: deprecate the old locking callbacks and add trylock.

Previously, there was no good way to request different kinds of lock
(say, read/write vs writeonly or recursive vs nonrecursive), or for a
lock function to signal failure (which would be important for a
trylock mode).

This patch revises the lock API to be a bit more useful.  The older
lock calls are still supported for now.

We also add a debugging mode to catch common errors in using the
locking APIs.

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