rpcs: simplify API for BlockService to handle multiple HTTP paths

[zeldovich]

Add a helper BlockService.RegisterHandlers() that registers handlers for multiple HTTP paths, and a RegisterHTTPHandlerFunc() method to GossipNode that can register handler functions in addition to handler objects, in preparation for adding more handlers (specifically, fetching a state proof).

[codecov]

Codecov Report

Attention: Patch coverage is 22.22222% with 14 lines in your changes missing coverage. Please review.

Project coverage is 56.27%. Comparing base (75bb6a9) to head (cddb78e).
Report is 3 commits behind head on master.

Files	Patch %	Lines
network/p2p/http.go	0.00%	5 Missing ⚠️
network/hybridNetwork.go	0.00%	3 Missing ⚠️
network/p2pNetwork.go	0.00%	2 Missing ⚠️
network/wsNetwork.go	0.00%	2 Missing ⚠️
rpcs/blockService.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5718      +/-   ##
==========================================
- Coverage   56.29%   56.27%   -0.02%     
==========================================
  Files         488      488              
  Lines       69581    69595      +14     
==========================================
- Hits        39172    39167       -5     
- Misses      27752    27771      +19     
  Partials     2657     2657              

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