mon: MonitorDBStore: allow randomly injecting random delays on writes

Adds two new config options:

mon_inject_transaction_delay_probability : DOUBLE (0.0-1.0, default: 0.0)
mon_inject_transaction_delay_max : DOUBLE (seconds, default: 10.0)

If probability is set to a value greater than 0, just before applying
the transaction, the store will decide whether to inject a delay,
randomly choosing a value between 0 and the max.

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>
mon: Paxos: reset accept timeout before submiting work to the store

Otherwise we may trigger the timeout while waiting for the work to be
committed to the store -- and it would only take the write to take a bit
longer than 10 seconds (default accept timeout).

We do wait for the work to be properly committed to the store before
extending the lease though.

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>