target: fix timer callbacks processing

Warning, behaviour change: before this patch if a timer callback
returned an error, the other handlers in the list were not called.

This patch fixes two different issues with the way timer callbacks are
called:

1. The function is not designed to be reentrant but a nested call is
possible via: target_handle timer event -> poll -> target events
before/after reexaminantion -> script_command_run ->
target_call_timer_callbacks_now . This patch makes function a no-op
when called recursively;

2. The current code can deal with the case when calling a handler
leads to its removal but not when it leads to removal of the next
callback in the list. This patch defers actual removal to consolidate
it with the calling loop.

These bugs were exposed by Valgrind.

Change-Id: Ia628a744634f5d2911eb329747e826cb9772e789
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2541
Tested-by: jenkins
Reviewed-by: Stian Skjelstad <stian@nixia.no>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>