server: Fix timeout for stream recording background jobs

[victorges]

What does this pull request do? Explain your changes. (required)
We noticed a couple errors that happen when saving transcoded segments for livestream recordings.
These occur especially in staging for some reason, but the underlying cause can easily happen in
production as well (and it often does).

In short terms, the issue is caused because saving the recorded segments is performed in a background
routine, while at the same time this routine uses the same context.Context to handle cancellation of the
task behind the scenes. This means that one the transcode request is finished (which is where the current
ctx comes from, the http.Request), the background saving operations will also be cancelled.

This fixes it by enforcing a separate timeout on the background saves, independent of the request.

Specific updates (required)

• Use a separate timeout context for the segment recording operations
• Increase the number of saving attempts to 3 instead of 2

How did you test each of these updates (required)
Automated tests. Pretty simple change as well, will check effect in staging on catalyst tests.

Does this pull request close any open issues?
livepeer/catalyst#126

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[codecov]

Codecov Report

Merging #2573 (7284abf) into master (a0ec61f) will increase coverage by 0.01618%.
The diff coverage is 66.66667%.

@@                 Coverage Diff                 @@
##              master       #2573         +/-   ##
===================================================
+ Coverage   56.41379%   56.42997%   +0.01617%     
===================================================
  Files             88          88                 
  Lines          18850       18857          +7     
===================================================
+ Hits           10634       10641          +7     
  Misses          7637        7637                 
  Partials         579         579                 

Impacted Files	Coverage Δ
clog/clog.go	56.00000% <0.00000%> (-1.37705%)	⬇️
server/broadcast.go	77.95414% <100.00000%> (+0.07802%)	⬆️
discovery/db_discovery.go	70.75812% <0.00000%> (+1.08303%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a0ec61f...7284abf. Read the comment docs.

Impacted Files	Coverage Δ
clog/clog.go	56.00000% <0.00000%> (-1.37705%)	⬇️
server/broadcast.go	77.95414% <100.00000%> (+0.07802%)	⬆️
discovery/db_discovery.go	70.75812% <0.00000%> (+1.08303%)	⬆️