Use 64-bit integer for session sending key counter on 32-bit platforms

[JeroenRobben]

The sending key/nonce counter must be 64 bits wide, but the current implementation uses AtomicUsize, which is only 32 bits wide on 32-bit platforms. This will wrap-around after 2^32 packets are sent in a single secure session, hence leading to nonce reuse. This is less likely in practice, since secure sessions are expected to rotate every 120 seconds.

This change switches the counter to AtomicU64. However, AtomicU64 is not available on arm-linux-androideabi (but is on the other arm targets), so maintaining support for that target may require using AtomicU64 from the portable_atomic crate or a Mutex<u64>