Avoid calling socket.peer_addr()

[msrd0]

Tokio's documentation says that calling .accept() in a loop is equivalent to using .incoming(), but it has the advantage of handing us the peer address together with the socket so we don't need to call .peer_addr() on the returned socket. Also, .incoming() has been removed in tokio 0.3.

Fixes #461

[codecov]

Codecov Report

Merging #497 into master will decrease coverage by 0.02%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##           master     #497      +/-   ##
==========================================
- Coverage   84.90%   84.88%   -0.03%     
==========================================
  Files         109      109              
  Lines        5599     5596       -3     
==========================================
- Hits         4754     4750       -4     
- Misses        845      846       +1     

Impacted Files	Coverage Δ
gotham/src/lib.rs	80.00% <82.35%> (-2.15%)	⬇️
gotham/src/middleware/session/backend/memory.rs	89.89% <0.00%> (-1.02%)	⬇️

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[nyarly]

This looks good. It reminds me that we'd like to handle protocol errors better, though.