Add log message when discarding recording segments in cache

[deviant77]

Currently Frigate silently discards recording segments in cache if there's more than "keep_count" for a camera.
This was happening on my system because it had a high load and was being slow to process the segments (copy segment and commit to the database).

I noticed when watching recordings that there was missing segments of video at random times, but I had no idea why because the segments were being discarded silently (even when logging set to debug).

This PR adds a warning log entry when discarding segments (in the same way as discarding corrupted segments) to aid troubleshooting.

[NickM-27]

Would probably be good to have a log message as well saying something like "deleting cached recording segments as {number of files} > {max_keep}" or something so it's clear why it's happening

[deviant77]

Would probably be good to have a log message as well saying something like "deleting cached recording segments as {number of files} > {max_keep}" or something so it's clear why it's happening

Good idea. I have done that.
Here's a sample of what the log output will look like:

[2022-07-18 23:21:52] frigate.record                 WARNING : Too many recording segments in cache for Kitchen. Keeping the 5 most recent segments out of 9, discarding the rest...
[2022-07-18 23:21:52] frigate.record                 WARNING : Discarding a recording segment: /tmp/cache/Kitchen-20220718232017.mp4
[2022-07-18 23:21:52] frigate.record                 WARNING : Discarding a recording segment: /tmp/cache/Kitchen-20220718232025.mp4
[2022-07-18 23:21:52] frigate.record                 WARNING : Discarding a recording segment: /tmp/cache/Kitchen-20220718232037.mp4
[2022-07-18 23:21:52] frigate.record                 WARNING : Discarding a recording segment: /tmp/cache/Kitchen-20220718232045.mp4