safekeeper: lift WAL preprocessing and filtering from pageserver

[erikgrinaker]

Following #9335, move preprocessing and shard filtering from pageserver to safekeeper.

This also requires protocol changes, where the subscriber specifies a shard ID, and receives preprocessed and filtered per-shard WAL records.

Protocol changes:

• Transmit ShardIndex to safekeeper when subscribing for WAL
• Change WAL send protocol to send ephemeral file ready buffer and list of metadata modification instructions
• Maintain application level checksumming. Currently, each wal record includes a crc, which is checked after the record is reconstructed on the pagserver side.