support passing headers and timeout in jrpc calls

[ldemailly]

fixes #619

sort of breaking api change, added some "jrpc.xxxxURL" backward compatible string url call points

[codecov]

Codecov Report

Base: 88.4% // Head: 88.4% // Increases project coverage by +0.1% 🎉

Coverage data is based on head (73c9268) compared to base (cbf33f7).
Patch coverage: 100.0% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #621     +/-   ##
========================================
+ Coverage    88.4%   88.4%   +0.1%     
========================================
  Files          30      30             
  Lines        4289    4309     +20     
========================================
+ Hits         3790    3810     +20     
  Misses        335     335             
  Partials      164     164             

Impacted Files	Coverage Δ
fhttp/http_server.go	80.8% <ø> (ø)
jrpc/jrpcServer.go	100.0% <ø> (ø)
rapi/restHandler.go	87.0% <ø> (ø)
fhttp/http_client.go	84.2% <100.0%> (ø)
fhttp/http_forwarder.go	90.2% <100.0%> (ø)
jrpc/jrpcClient.go	100.0% <100.0%> (ø)

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[wiardvanrij]

Basically only a few nits/questions