Remove the arbitrary time before answering when calling from the switchboard

The new behavior uses the ExtensionStatus event to answer the phone only
after it starts ringing.
move the answer the next call logic from the user_manager to the call_manager
stop using a timer to answer when retrieving a call on hold using the switchboard
use the new SIPRinging event to trigger the switchboard answer
add a 0.25 sec sleep before answering a snom switchboard

The device probably reply with a 180 ringing before it starts ringing.
Since this delay is on the phone side, it should not vary that much
from installation to installation.
rename method get_answer_on_exten_status_fn

the new callback is on the SIPRinging event, hence the new name
_get_answer_on_sip_ringing_fn
Revert the incomplete rename of _get_answer_on_exten_status_fn

This reverts commit c2ea2bbcdbe5c51ce0494787dd847c82b1e27cfa.
add an env variable SNOM_SB_ANSWER_DELAY

To make troubleshooting easier the SNOM_SB_ANSWER_DELAY can be used to
change the default delay before a snom switchboard tries to answer a call.
rename method _get_answer_on_exten_status_fn

the new callback is on the SIPRinging event, hence the new name
_get_answer_on_sip_ringing_fn
make the snom default answer delay 0.5s

This is the same value as before. To avoid breaking working installation