Try to fix errors of using WebSockets closed from client side

[serhiy-storchaka]

• The "top" handler now listen the web socket and handles pings and closing the connection from client side. It allows to simple close the connection from client side (Cannot properly close ws connection in one-sided way. aio-libs/aiohttp#3443).
• The "top" and the "log_ws" handlers now check whether the web socket was closed before reading from or writing to it. It saves us from numerous error records in the service logs. Similar issues may be fixed in other handlers (port forward, attach), although I did not see such log records for them (they should be more rare).
• The "log_ws" handler no longer sets WS protocol "v2.channels.neu.ro", as it is not compatible with it.
• Some refactoring.
• Fixed some minor errors and typos.

[codecov]

Codecov Report

Merging #737 (1a963cf) into master (1a963cf) will not change coverage.
The diff coverage is n/a.

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##           master     #737   +/-   ##
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  Coverage   85.86%   85.86%           
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  Files          12       12           
  Lines        2080     2080           
  Branches      318      318           
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  Hits         1786     1786           
  Misses        187      187           
  Partials      107      107           

Flag	Coverage Δ
integration	81.00% <0.00%> (ø)
unit	46.05% <0.00%> (ø)

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