The connections to APN are the only connections in pling that are
steadily held open (Apple requires this).

Without a connection pool there might be race-conditions when using
pling in a multi-threaded context.

This adds a connection pool so that only one thread at a time can use an
open connection to APN.

Instead of caching the connections in the C2DM and GCM gateways in
instance variables, we use the thread local storage provided by
`Thread.current`.

The reason for that is thread safety, since gateway objects are
practically singleton objects that can be used by multiple threads at
the same time.

With this change each thread that calls `deliver` on a gateway gets a
fresh connection object and doesn't interfer with the other threads'
state.

Using Thread.current is basically using a hidden global variable. This
is simpler and safer.