mon: MonitorDBStore: allow randomly injecting random delays on writes

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>
(cherry picked from commit beaa04e4119765d5775a6c48fd072dd95c984e3b)