Reorganize pair converter code

[eprbell]

This is preparatory work for my experiments writing a CCXT-based version of the Coinbase plugin (which has been broken by Coinbase, as described in #216).

This PR moves graph/optimization code from abstract_pair_converter_plugin.py to abstract_ccxt_input_plugin.py (no functional change). This is because:

• the code was interfering with simple plugins like Historic-Crypto, which was printing the following message: "No Fiat Access Key. Fiat pricing will NOT be available. To enable fiat pricing, an access key from exchangerate.host is required.". This was confusing because Historic-Crypto has no dependency on exchangerate.host.
• it's cleaner to have a bare bone top class (abstract_pair_converter_plugin.py) and a graph/optimization-aware subclass (abstract_ccxt_input_plugin.py). This way, simple plugins don't have to inherit the extra complexity of optimization.

This PR also contains minor changes to exception handling: ExchangeError doesn't need to retry.

[macanudo527]

@eprbell Sorry, I did see this. I just need to patch code to see if it works. (Right now I can't get accurate prices because things are out of date).

I had to update the fiat pricing and kraken pricing. I think I have it now. I'm going to do some cleaning and submit it. I might need to skip writing tests for now, though. Trying to get it all done before the tax deadline.

[eprbell]

@eprbell Sorry, I did see this. I just need to patch code to see if it works. (Right now I can't get accurate prices because things are out of date).

I had to update the fiat pricing and kraken pricing. I think I have it now. I'm going to do some cleaning and submit it. I might need to skip writing tests for now, though. Trying to get it all done before the tax deadline.

No worries! Thanks for reviewing my PR: it's good to turn things around :-)

[macanudo527]

@eprbell did you mean to create a separate cache for CCXT and the abstract pair plugin? in CCXT there is self._cache, but the abstract has self.__cache

[eprbell]

@eprbell did you mean to create a separate cache for CCXT and the abstract pair plugin? in CCXT there is self._cache, but the abstract has self.__cache

I think the _cache property should probably be in the top abstract class (i.e. abstract_pair_converter_plugin.py) and be inherited by all subclasses including CCXT-based ones.

[macanudo527]

I think the _cache property should probably be in the top abstract class (i.e. abstract_pair_converter_plugin.py) and be inherited by all subclasses including CCXT-based ones.

OK, that's what I was thinking. I'll fix my code in #225 to reflect that.