Move key prefixing logic from command classes to prefix processor.

While command classes define how the client should filter arguments or
parse responses, key prefixing depends on the actual command signature
as defined by Redis so it really is something that should be handled
separately as the norm.

Developers can define new handlers or override existing ones, but they
can still define the key prefixing logic inside their command classes
by implementing Predis\Command\PrefixableCommandInterface: the key
prefix processor will just use that by overriding any defined handler.
Merge branch 'v0.9/external-prefixer'
[tests] Add "count" argument to SRANDMEMBER (since Redis >= 2.6).
[tests] Add missing tests for SETNX.

For some obscure reason Predis/Command/StringSetPreserve had no tests.
[tests] Improve base command test case.
[tests] Improve tests of TTL.