cache accounts() and instruments() requests

Until now the cacheProxy class was only able to cache history() requests
to a server. Since we got support for invalidating our cached data, this
change can now add support for also caching accounts() and instruments()
requests.
Typically, the data returned by these requests would rarely change (if
at all), so caching can provide significant benefits if the results are
used often.
first step towards profit/loss calculation
continue work
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