[mythtv-cmyth] Review recordings UID and how handle recording events

Many problems with it and myth protocol changes. So to resolve for long term
i rationalized the UID. Now it is formatted like "1002 2013-08-23T14:42:00Z"
and contains myth channel id and recstartts as UTC time. Regardless myth
version our UID use UTC time only.
Depending of protocol command, backend messages could be filled with time
values containing Unix time or string using ISO8601 standard. The string can
contains UTC or local time. To process recording events from backend we must
match backend message with recording UIDs.

These changes allow to resolve failure of update file size events and  to clear
tricky code.

Cosmetics around recording events
- New function to handle a particular event
- New virtual function MythObserver::UpdateRecordings()
- New function to manage timeout or trigger update recordings
[mythtv-cmyth] Remove useless code

Since commit 035cff0a1c5ccd56ea88860d324109798dfd127b
[cmyth] Cosmetics

- Function cmyth_timestamp_from_tm() is local. Fix storage type as static.
- Function cmyth_timstamp_diff() should return number of seconds. Use difftime() from standard lib.
[mythtv-cmyth] Cosmetics around time_t type

- time_t is undefined arithmetic type. In GetEPGForChannel(), convert startTime to number of minutes since epoch before cast.
- Check times of PVRTimer using time(). If original values equal 0 (NULL), time(NULL) will return current time as expected.
- Fix timeslot for all.