Handle server initiated global requests

[pepijnve]

RFC 4254 section 4 states that either side of the connection
may send global requests at any time and that the receiver
MUST respond appropriately.

This PR replaces the default logic of responding with 'UNIMPLEMENTED'
with the correct 'MSG_REQUEST_FAILURE' if 'wantReply' is set.

[codecov-io]

Codecov Report

Merging #472 into master will decrease coverage by 0.07%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #472      +/-   ##
============================================
- Coverage     55.92%   55.84%   -0.08%     
- Complexity     1215     1216       +1     
============================================
  Files           192      192              
  Lines          7859     7870      +11     
  Branches        714      715       +1     
============================================
  Hits           4395     4395              
- Misses         3112     3123      +11     
  Partials        352      352

Impacted Files	Coverage Δ	Complexity Δ
...va/net/schmizz/sshj/connection/ConnectionImpl.java	58.06% <0%> (-5.66%)	20 <0> (ø)

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[hierynomus]

Thanks @pepijnve!

[pepijnve]

Context for this change is that I kept seeing Sending SSH_MSG_UNIMPLEMENTED debug messages scroll by which led me to wonder what wasn't implemented exactly.
Turns out on of the servers I was connected to was sending keep-alive@openssh.com global requests periodically. After this change those get handled silently.

[hierynomus]

Thanks for the background info!