Avoid rounding errors in semaphore handling

lock_acquire() adds 0.001 (1 ms) to the timeout “in order to avoid errors with float to string conversion”. This was mentioned in #251792-155: Implement a locking framework for long operations comment #155, but without any explanation of why 1 ms is necessary and sufficient to avoid the rounding errors. Why not 0.001 ms or 0.5 second?

It seems that in D7 1 ms is enough, but in D6 it is not. It seems that the difference is due to how floating point numbers are handled inside the PDO driver (?!).

On D6, try inserting the following row:

INSERT INTO semaphore VALUES ('foo','20235236224d7df493403f85.14852477',1300100273.2631);

Notice that lock_may_be_available('foo') returns FALSE, even though the timeout is in the past, so the function is supposed to delete that row. The semaphore gets stuck forever.

This patch changes the code to use the current time rather than the looked up timeout when deleting expired semaphores. This should be just as correct, and it makes the code more robust (so we don't need to research if 1 ms really is enough to avoid the rounding error).