Fix: consumer metadata switch timer error handling

The thread handling the metadata switch cannot call
consumer_timer_switch_stop(), because it would become stucked waiting
for itself to execute the teardown signal handler.

Moreover, only one thread is allowed to call start/stop timer (by
design). Therefore, it's a race to have the thread handling the timer
handler calling "stop".

Fix this by adding a "switch_timer_error" flag, which just inhibits the
timer if an error is encountered, but does not modify its state
otherwise.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>