Pixmap issues with backends that go away

[gdragon at jetcom dt org]

If you have slave backends that aren't always running, either broken, or not always attached, the pixmap generation will regenerate pixmaps all the time.

Using MasterBackendOverride, the recordings really available on the master backend, but if the slave is not around to handle the QUERY_PIXMAP_LASTMODIFIED it will return bad unnecessarily. At least in mythweb's case, this will kick off a QUERY_GENPIXMAP even though there is already a good pixmap there.

This behavior is most noticeable when mythweb takes a long time to load the recorded page, as it regens the pixmaps for all [offline] slave's recordings, everytime.

Attached patch will try and look locally for file info if the override is on. I left the code in there to check for slaves first, so if there is a slave online, the slave is responsible for the check. It might be marginally more efficient to let the master check but I didn't bother to fix that.

[Isaac Richards]

(In [10043]) Let the master backend answer pixmap requests for slaves if the MBO is on.

Closes #1862.