Enforce non-empty ASPath on eBGP sessions and set local-pref 100 if none is set

[BarbarossaTM]

This does two things:

Hide routes received via eBGP when the AS-Path is empty

I was wondering if it would be a good idea to just create an AS-Path with the peer ASN in it, but depending on what the peer did to their policies this may not reflect reality and may actually further support leaking prefixes or something. So I decided to err on the side of caution here and consider prefixes received via eBGP and w/o an AS-Path as invalid/hidden. We may want to ignore this check when learning prefixes via BMP, however I would hope that no BGPd out there would accept it.

Default local-pref for eBGP prefixes to 100

This is a follow-up on a previous PR, which only took effect when running BIO as BGP server and not as library. This fixes the residual places.

[codecov-commenter]

Codecov Report

Merging #449 (3892a5f) into master (2f55bf4) will increase coverage by 0.01%. The diff coverage is 66.66%.

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@@            Coverage Diff             @@
##           master     #449      +/-   ##
==========================================
+ Coverage   54.33%   54.34%   +0.01%     
==========================================
  Files         146      146              
  Lines        7894     7901       +7     
==========================================
+ Hits         4289     4294       +5     
- Misses       3400     3401       +1     
- Partials      205      206       +1     

Flag	Coverage Δ
unittests	54.34% <66.66%> (+0.01%)	:arrow_up:

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Files Changed	Coverage Δ
protocols/bgp/server/fsm_address_family.go	73.41% <ø> (ø)
route/path.go	84.79% <0.00%> (-1.01%)	:arrow_down:
routingtable/adjRIBIn/adj_rib_in.go	59.85% <85.71%> (+1.52%)	:arrow_up:

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