Fix unintentional creation of defaultdict key by __getitem__ calls

[ezdac]

Before, the whitelisting mechanism of the services had a bug:

Several __getitem__ calls meant to be readonly to the internal defaultdict that tracks the whitelisted user-addresses were unintentionally causing the address being added to the dict with a default-value.
This in the end let the address pass the whiltelisting check:

raiden-services/src/raiden_libs/user_address.py

Lines 311 to 312 in 091c6ca

	if address is None or not self.is_address_known(address):
	return

Which can potentially lead to performance issues and heavy load on the matrix servers.

This PR fixes this issue.

[codecov]

Codecov Report

Merging #1110 (3e4004b) into master (091c6ca) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1110      +/-   ##
==========================================
- Coverage   88.80%   88.71%   -0.10%     
==========================================
  Files          43       43              
  Lines        3449     3447       -2     
  Branches      430      429       -1     
==========================================
- Hits         3063     3058       -5     
- Misses        279      282       +3     
  Partials      107      107              

Impacted Files	Coverage Δ
src/raiden_libs/user_address.py	74.70% <100.00%> (-0.20%)	⬇️
src/raiden_libs/blockchain.py	84.21% <0.00%> (-0.88%)	⬇️
src/pathfinding_service/api.py	93.63% <0.00%> (-0.64%)	⬇️

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[palango]

Nice find! 🏅

Can you, while you're at it also lower the codecov coverage target: https://github.com/raiden-network/raiden-services/blob/master/.codecov.yml#L6 to something we hit, e.g. 88?