cmd/bootnode, p2p: support for alternate mapped ports

[ucwong]

This changes the port mapping procedure such that, when the requested port is unavailable an alternative port suggested by the router is used instead.

We now also repeatedly request the external IP from the router in order to catch any IP changes.

[codecov]

Codecov Report

Merging #1635 (f7ea572) into master (e0f2156) will increase coverage by 0.06%.
The diff coverage is 45.97%.

❗ Current head f7ea572 differs from pull request most recent head 746c26d. Consider uploading reports for the commit 746c26d to get more accurate results

@@            Coverage Diff             @@
##           master    #1635      +/-   ##
==========================================
+ Coverage   52.53%   52.59%   +0.06%     
==========================================
  Files         480      481       +1     
  Lines       59963    60210     +247     
==========================================
+ Hits        31501    31669     +168     
- Misses      26080    26142      +62     
- Partials     2382     2399      +17     

Impacted Files	Coverage Δ
cmd/utils/flags.go	1.84% <0.00%> (-0.01%)	⬇️
core/state/journal.go	72.54% <0.00%> (-2.97%)	⬇️
core/state/snapshot/conversion.go	0.00% <0.00%> (ø)
core/state/snapshot/generate.go	0.00% <0.00%> (ø)
core/state/snapshot/journal.go	0.00% <0.00%> (ø)
core/state/snapshot/snapshot.go	41.06% <0.00%> (-0.59%)	⬇️
core/types/state_account.go	0.00% <0.00%> (ø)
core/vm/gas_table.go	11.88% <ø> (+0.24%)	⬆️
core/vm/memory_table.go	0.00% <0.00%> (ø)
core/vm/opcodes.go	18.18% <ø> (ø)
... and 23 more

... and 11 files with indirect coverage changes