nextTick: Preserve depth in error/reentry cases

When there is an error that is thrown in a nextTick function, which is
then handled by a domain or other process.on('uncaughtException')
handler, if the error handler *also* adds a nextTick and triggers
multiple MakeCallback events (ie, by doing some I/O), then it would
skip over the tickDepth check, resulting in an infinite spin.

Solution: Check the tickDepth at the start of the tick processing, and
preserve it when we are cleaning up in the error case or exiting early
in the re-entry case.

In order to make sure that tick callbacks are *eventually* handled, any
callback triggered by the underlying spinner in libuv will be processed
as if starting from a tick depth of 0.