Add a whole balance limit to UserDeposit

[pirapira]

which is configurable at the deployment time.

Currently we keep the option of limiting deposits in all smart contracts (https://github.com/raiden-network/team/issues/324#issuecomment-465100567).

I chose against using the contract's token balance because that poses a DOS attack possibility.

This closes #585.

[karlb]

I'm not sure which requirements we have for the choice of our limits. Having a per-account limit would be simpler and more gas efficient than having a global limit. But I assume you know that and did it this way for a good reason.

[pirapira]

@karlb the reason for the total limit can be seen here: https://github.com/raiden-network/team/issues/333

In general, one person can create an unlimited number of Ethereum accounts, so usually per-account limitations are not very effective.

[codecov]

Codecov Report

Merging #678 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #678   +/-   ##
=======================================
  Coverage   20.29%   20.29%           
=======================================
  Files          16       16           
  Lines        1069     1069           
  Branches      118      118           
=======================================
  Hits          217      217           
  Misses        850      850           
  Partials        2        2

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[codecov]

Codecov Report

Merging #678 into master will increase coverage by 44.49%.
The diff coverage is 83.33%.

@@             Coverage Diff             @@
##           master     #678       +/-   ##
===========================================
+ Coverage   20.29%   64.79%   +44.49%     
===========================================
  Files          16       17        +1     
  Lines        1069     1122       +53     
  Branches      118      119        +1     
===========================================
+ Hits          217      727      +510     
+ Misses        850      369      -481     
- Partials        2       26       +24

Impacted Files	Coverage Δ
raiden_contracts/deploy/__main__.py	67.75% <83.33%> (+67.75%)	⬆️
setup.py	0% <0%> (ø)
raiden_contracts/utils/private_key.py	25% <0%> (+25%)	⬆️
raiden_contracts/utils/signature.py	88% <0%> (+32%)	⬆️
raiden_contracts/utils/transaction.py	65.21% <0%> (+43.47%)	⬆️
raiden_contracts/utils/proofs.py	91.89% <0%> (+45.94%)	⬆️
raiden_contracts/contract_manager.py	78.16% <0%> (+55.63%)	⬆️
raiden_contracts/utils/merkle.py	82.35% <0%> (+55.88%)	⬆️
raiden_contracts/utils/logs.py	87.5% <0%> (+68.26%)	⬆️
... and 3 more

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