Add manual batching mechanism

[jean-roland]

Closes #298. This adds an API to do manual batching which yields improved throughput performance for smaller packets.

User can start batching with zp_start_batching, any subsequent api call that would have put a network message (z_put, z_get, z_declare...) is bufferized to be later sent as a batch (or multiple batches) when user calls zp_stop_batching.

While I was modifying the transports I decided to refactor a little because the functions were too complicated to parse.

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