Fix #588 Wallet list index out of range error

[AlexCato]

Uses the number of mixdepths from the wallet file

[coveralls]

Coverage decreased (-0.09%) to 79.945% when pulling 141be19 on AlexCato:fix_wallet_index_error into 984626b on JoinMarket-Org:develop.

[AlexCato]

I think I found a problem with this solution:

Given there's a standard wallet with 5 mixdepths, but you start a yieldgenerator with a max_mixdepth-configuration of fewer than 5: then there will be wallet.json-file will be changed to contain the fewer index caches.
This PR then will make it impossible to ever be able to use the original number of mixdepths again.

Proposed solution:
Changing the default value of max_mixdepth in the configuration to -1 (from 5) and only read the number of mixdepths from the wallet file if the user does not specify the max_mixdepth.
Will code that later if noone sees a problem with that.

[AlexCato]

Changed default in wallet-tool.py to -1 now. This will auto-detect the number of mixdepths only if the user does not explicitly configure it.

Therefor the default behavior is improved without any change in behavior if the user intentionally sets the parameter.

[coveralls]

Coverage decreased (-0.08%) to 79.958% when pulling b7db6d1 on AlexCato:fix_wallet_index_error into 984626b on JoinMarket-Org:develop.

[chris-belcher]

utACK on this. Anyone else have any thoughts?

[AdamISZ]

Agreed. No conceivable negative I can see. Merging. Edit: on second thoughts, need to think a bit :)