Re-organize the client module

Re-organize the client module, creating a `gateway` submodule, and
splitting the connection into separate files in it.

The connection was a conglomeration of a number of purposes, most of
which are actually used elsewhere in the library and/or exposed to the
user. Thus, it makes sense to separate each item in a gateway-specific
module.

By splitting the client module further, this is a re-organization for
preliminary RPC support WRT the Client.

Additionally, rename the Connection struct to a Shard. The Connection
itself was not the actual connection, and was a higher-level interface
to the real connection logic. A Shard is a more accurate representation
of what it actually is.

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