log-slave-updates and circul repl

Slave SQL thread may execute one extra event when there are events
skipped by slave I/O thread (e.g. originated by the same server).
Whereas it was requested not to do so by the UNTIL condition.

This happens because we compare with the end position of previously
executed event. This is fine when there are no skipped by slave I/O
thread events, as end position of previous event equals to start
position of to be executed event. Otherwise this position equals to
start position of skipped event.

This is fixed by:
- reading the event to be executed before checking if the until condition
  is satisfied.
- comparing the start position of the event to be executed. Since we do
  not have the start position available, we compute it by subtracting
  event length from end position (which is available).
- if there are no events on the event queue at the slave sql starting
  time, that meet until condition, we stop immediately, as in this
  case we do not want to wait for next event.