Remove (broken) support for floating point user keys

[jhecking]

The client currently accepts float values (as well as integers, strings and Buffers) as record keys. However float values are being truncated to integers before the key digest is calculated! Therefore float values are basically useless as record keys since e.g. the key 3.14159 and the key 3 would both generate the same digest and therefore read/write the same record.

Since the server officially only supports integers, strings and byte arrays (Buffers) as record keys, the client should reject floating point record keys.