Change MythTimer to use monotonic clock

[Rune Petersen <rune@…>]

Currently MythTimer? is implemented using QTime which relies on system time and timezone conversion have the following problems:

• imprecise if system time is changed
• imprecise if if a change to/from DST happens

The patch uses QElapsedTimer instead of QTime. QElapsedTimer relies on monotonic clock, "get ticks" or similar.

Known problems with this patch:

• QElapsedTimer requires QT 4.7 (what is currently the minimum requirement?)
• addMSecs() cannot be implemented with QElapsedTimer, but then again the current implementation is a no-op.

This patch reduces CPU usage by ~5% on a 700MHz Cortex-A9 when recording DVB HD content.

[Daniel Thor Kristjansson <danielk@…>]

In 440573f3febdc8009b74ae1b2366aa7d518a15a7/mythtv:

Convert MythTimer? to use QElapsedTimer instead of QTime.

Fixes #11255.

We initially created MythTimer? because QTime isn't monotonic
and we wanted a wrapper that assured that time wouldn't march
backward too much. As of Qt 4.7 there is a timer available
which is monotonic on most systems. This converts MythTimer?
to use this new timer.

The first version of this new wrapper was written by
Rune Petersen for the efficiency gains this has over
QTime which does an expensive DST conversion internally.
I added back a timer restart in the elapsed() method
since QElapsedTimer doesn't always provide a monotonic
timer, and I implemented the addMSecs() method so this
could be used in all instances where the original
MythTimer? was used.

[Daniel Thor Kristjansson <danielk@…>]

In 94cbafa832d42245bec279fd56ca4416d75c2ce2/mythtv:

Add unit testing for MythTimer?. Refs #11255.

[Rune Petersen <rune@…>]

Thank you Daniel

[Daniel Thor Kristjansson <danielk@…>]

In 926464a3f11448689523db3441515ef3c6bd8ce8/mythtv:

Fix a misuse of MythTimer?.

This was what forced the [94028580] workaround earlier this week.

Refs #11255.