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.
introduce EMA strategy
MERGE W/ c96a208621f2b96a56b4d1d620e9891183e71369
MERGE W/ c96a2086
MERGE W/ f21e8253
enhance EMA calculation
continue work