More rubust cache keys

[morloderex]

When determining a cache key and stating that the ttl for the query should be now until for instance the end of day. We have a little tiny bug in that the sha1 key will ALWAYS be different sense ttl value is stored alongside the cache key. Use to the serialization of the object.

This pr fixes that by being more explicit about how the sha1 value is created now instead of serializing the whole exchangeQuery object we take out the specific parameters which we know are constants.

E.g the base and counter currency and for historical fetches we also take out the date in Y-m-d format

[florianv]

@morloderex Thanks for your PR :) I would maybe instead introduce a CacheResolver so everyone could customize the strategy based on their caching needs and pass it as a constructor param to Exchanger, with the default resolver being the current one to keep B/C

[morloderex]

@florianv thanks for your feedback.
That's a great idea, i will make the changes for that one. But would you consider it as a breaking change if i change the constructor?