Geth stop using txpool api

[LefterisJP]

Fixes: #4976

PR review check list

Quality check list that cannot be automatically verified.

• Safety
  • The changes respect the necessary conditions for safety (https://raiden-network-specification.readthedocs.io/en/latest/smart_contracts.html#protocol-values-constraints)
• Code quality
  • Error conditions are handled
  • Exceptions are propagated to the correct parent greenlet
  • Exceptions are correctly classified as recoverable or unrecoverable
• Compatibility
  • State changes are forward compatible
  • Transport messages are backwards and forward compatible
• Commits
  • Have good messages
  • Squashed unecessary commits
• If it's a bug fix:
  • Regression test for the bug
    • Properly covers the bug
    • If an integration test is used, it could not be written as a unit test
• Documentation
  • A new CLI flag was introduced, is there documentation that explains usage?
• Specs
  • If this is a protocol change, are the specs updated accordingly? If so, please link PR from the specs repo.
• Is it a user facing feature/bug fix?
  • Changelog entry has been added

[LefterisJP]

This is succesfully tested by the following test:

raiden/raiden/tests/integration/rpc/test_client.py

Line 7 in 114f5a4

def test_geth_discover_next_available_nonce(

[codecov]

Codecov Report

Merging #4978 into develop will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #4978      +/-   ##
===========================================
+ Coverage    80.66%   80.69%   +0.02%     
===========================================
  Files          120      120              
  Lines        14606    14594      -12     
  Branches      2264     2260       -4     
===========================================
- Hits         11782    11776       -6     
  Misses        2156     2156              
+ Partials       668      662       -6

Impacted Files	Coverage Δ
raiden/ui/checks.py	54.76% <ø> (ø)	⬆️
raiden/network/rpc/client.py	76.53% <100%> (+0.38%)	⬆️
raiden/network/transport/matrix/client.py	68.5% <0%> (-1.3%)	⬇️
raiden/network/proxies/token_network.py	50.42% <0%> (-0.74%)	⬇️
raiden/api/python.py	68.33% <0%> (-0.53%)	⬇️
raiden/transfer/channel.py	89.21% <0%> (-0.23%)	⬇️
raiden/network/transport/matrix/transport.py	80.94% <0%> (+0.91%)	⬆️
raiden/waiting.py	85.71% <0%> (+4%)	⬆️
raiden/storage/serialization/types.py	68.65% <0%> (+4.47%)	⬆️

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[rakanalh]

LGTM