trickle: Sweep idle channels on the server.

[j0sh]

Eventually cleans up anything on the trickle server side in case we miss a cleanup somewhere.

This does imply that channels need to be active regularly, so for naturally intermittent channels like control or events we will have to send a keepalive via clients periodically. (keepalives will also be useful for keeping client connections alive.)

Default 5-minute idle timeout, with a 1-minute sweep interval (both configurable). This basically requires us to explicitly start the trickle server (in order to start sweeping), and the Start() function returns a cancel-type function in order to stop things, a'la context cancel.

Also adds an explicit "channel create" endpoint to the trickle server (empty POST to /channel-name without an associated sequence number). Not used right now but it has come in handy while testing elsewhere, eg in python.

Also includes this change to make the startAIServer take a *lphttp so we aren't copying the entire struct every time we pass this in, and can make changes to its fields (eg, assigning the trickle server)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 71 lines in your changes missing coverage. Please review.

Project coverage is 34.14555%. Comparing base (51fd713) to head (8b71572).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
trickle/trickle_server.go	0.00000%	57 Missing ⚠️
server/ai_http.go	0.00000%	10 Missing ⚠️
server/rpc.go	0.00000%	4 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3296         +/-   ##
===================================================
- Coverage   34.19096%   34.14555%   -0.04541%     
===================================================
  Files            139         139                 
  Lines          36846       36895         +49     
===================================================
  Hits           12598       12598                 
- Misses         23529       23578         +49     
  Partials         719         719                 

Files with missing lines	Coverage Δ
server/rpc.go	67.26190% <0.00000%> (-0.60597%)	⬇️
server/ai_http.go	10.25641% <0.00000%> (ø)
trickle/trickle_server.go	0.00000% <0.00000%> (ø)

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Files with missing lines	Coverage Δ
server/rpc.go	67.26190% <0.00000%> (-0.60597%)	⬇️
server/ai_http.go	10.25641% <0.00000%> (ø)
trickle/trickle_server.go	0.00000% <0.00000%> (ø)

[j0sh]

Holding off on merging this until we have keepalives to ensure control API liveness since messages there are intermittent and we don't want those to be swept up

[j0sh]

Once #3299 and livepeer/ai-worker#313 are in, I'll merge this one