Expose MEMORY USAGE to the python client

[ctandon11]

Description

Give the python client access to the Redis command MEMORY USAGE by exposing it
through the Pybindings.

Justification

The user will be able to get the number of bytes that a key and its value
require to be stored in RAM. The user will be able to track their memory
usage better.

Implementation Strategy

MEMORY USAGE will be a SingleKeyCommand.

The implementation will look something like this in client.cpp:

uint64_t Client::memory_usage(std::string key)
{
    SingleKeyCommand cmd;
    cmd->add_field("MEMORY");
    cmd->add_field("USAGE");
    cmd->add_field(key, true)

    CommandReply reply = _run(cmd);
    if (reply.has_error() > 0)
        throw std::runtime_error("LATENCY RESET command failed");

    return reply.integer();
}

This functionality then needs to be exposed through the Pybindings and pyclient.cpp file to the python client layer.