Add support for writing/reading arbitrary values not just floats

[robskillington]

Currently all the interfaces both at RPC and the package level only allow for float64 values to be written and read into M3DB.

There is no need for such such a restriction, as long as users can provide a stream encoder and decoder there's no reason why they can't write/read arbitrary data structures to M3DB for time series data.

Perhaps we can just add some generic write/read methods alongside the current specialized float64 methods.