Optimize Agent, fix bugs

[szmarczak]

Some tests are timing out. Will fix soon. Fixed.

Why the change? Well, busy sessions are very rare. It's not worth separating them into another object.

[szmarczak]

TODO:

• test graceful close,
• _freeSessionsCount, _sessionsCount,
• NGHTTP2_REFUSED_STREAM on www.facebook.com sindresorhus/got#1285

[codecov-commenter]

Codecov Report

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The diff coverage is n/a.

@@            Coverage Diff             @@
##             master       #42   +/-   ##
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  Coverage          ?   100.00%           
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  Files             ?         7           
  Lines             ?       575           
  Branches          ?         0           
==========================================
  Hits              ?       575           
  Misses            ?         0           
  Partials          ?         0           

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