Workaround for web3 throwing exception in

[pirapira]

web3.eth.generateGasPrice() when there are not many transactions in the chain.

This fixes #3244 on the procedure documented in
https://github.com/raiden-network/raiden/blob/fe45769dd3c3f339b30a214599d028eb8351dc6a/docs/private_net_tutorial.rst

[codecov]

Codecov Report

Merging #3810 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3810      +/-   ##
==========================================
+ Coverage   77.61%   77.61%   +<.01%     
==========================================
  Files         109      109              
  Lines       14833    14839       +6     
  Branches     2074     2076       +2     
==========================================
+ Hits        11512    11517       +5     
- Misses       2613     2614       +1     
  Partials      708      708

Impacted Files	Coverage Δ
raiden/network/rpc/client.py	63.23% <0%> (-0.33%)	⬇️
raiden/waiting.py	79.54% <0%> (-1.41%)	⬇️
raiden/transfer/mediated_transfer/target.py	98.43% <0%> (-0.79%)	⬇️
raiden/transfer/events.py	74.9% <0%> (-0.78%)	⬇️
raiden/network/transport/matrix/transport.py	77.97% <0%> (-0.15%)	⬇️
raiden/transfer/mediated_transfer/mediator.py	89.18% <0%> (+0.62%)	⬆️
raiden/transfer/mediated_transfer/events.py	84.25% <0%> (+0.92%)	⬆️
raiden/network/transport/udp/udp_utils.py	84.74% <0%> (+3.38%)	⬆️

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