Bug 889245 - [Call log] When user make a voicemail call, call log shows only number

Implement some caching into Voicemail.check now that we use it for every number in the call log.
Merge branch 'bug889245' into bug890205
Bug 890205 - Display emergency number string in call log

This implies some database changes, since we can only determine whether
a phone number is an emergency one when making that call. Exposing this
to Gaia has been refused many times. So instead we add an emergency
field in the call log and when we save the call made we take the chance
to remember that it was an emergency number.

The displaying logic is a bit different from the Voicemail case,
especially for the same reason: we have to pull the information out of
the database. Once we have it (i.e., when we are in updateContactInfo),
we apply the same logic than for voicemail, the only difference being
that the emergency flag is pulled from dataset rather than params.