Allow configuring http client timeout

[yurishkuro]

Which problem is this PR solving?

• Users are unable to configure http client timeout

Short description of the changes

• Added support for JAEGER_REPORTER_TIMEOUT environment variable. Updated docs. Added tests.
• Replaces abandoned WIP: Allow configuring http client timeout #350

[codecov]

Codecov Report

Merging #465 (d9efad0) into master (8e6f4ec) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #465   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files          50       50           
  Lines        2039     2044    +5     
  Branches      384      386    +2     
=======================================
+ Hits         2013     2018    +5     
  Misses         26       26           

Impacted Files	Coverage Δ
src/configuration.js	99.00% <100.00%> (+0.02%)	⬆️
src/configuration_env.js	100.00% <100.00%> (ø)
src/reporters/http_sender.js	100.00% <100.00%> (ø)

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