Batch pushed WAL frames

[penberg]

We currently send one frame at a time, which has lots of overhead from TLS handshake, for example. Let's send WAL frames in batches -- perhaps make it configurable and default to 32 KiB (which is 8 frames). We could also make batching smart and send full transactions at at time (or multiple transactions). The server side is built to allow partial transactions to be sent, but sending full transactions has less overhead.