transport: make _requests_serving param uint32_t
transport: make _requests_serving param uint32_t

It's not realistic for a shard to have over 4 billion concurrent
requests, so this value can be safely represented in 32 bits.
Also, since the current concurrency limit is represented in uint32_t,
it makes sense for these two to have matching types.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla