BMP Receiver: Close Connection Explicitly and with log message

[margau]

It was not clear why a connection was not properly accepted after the connection came from the wrong source. This PR changes the behaviour by explicitly closing the BMP TCP Connection and adding a log output.

[codecov-commenter]

Codecov Report

Merging #417 (9a7d889) into master (ecd3fda) will decrease coverage by 0.07%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #417      +/-   ##
==========================================
- Coverage   54.53%   54.47%   -0.07%     
==========================================
  Files         147      147              
  Lines        7879     7885       +6     
==========================================
- Hits         4297     4295       -2     
- Misses       3377     3384       +7     
- Partials      205      206       +1     

Flag	Coverage Δ
unittests	54.47% <0.00%> (-0.07%)	:arrow_down:

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Impacted Files	Coverage Δ
protocols/bgp/server/bmp_receiver.go	9.45% <0.00%> (-0.30%)	:arrow_down:
protocols/device/server.go	78.72% <0.00%> (-4.26%)	:arrow_down:

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[taktv6]

Looks good. Thanks for your contribution!