Send not_found for unservable get_data.

[echennells]

A get_data naming a block or transaction the node cannot serve stops the channel. bitcoind replies notfound and stays connected. Implements #986.

Three sites now reply and resume the send loop: a pruned block, a block absent from the archive, and an absent transaction. Each item is answered on the send completion handler, as a block is, so nothing is produced until the prior write completes.

Block-out protocols are selected on send_headers, so a peer above bip37 reaches protocol_block_out_106 through the default arm. not_found is undefined below bip37, so that protocol captures the negotiated level at construction and retains the stop below it. protocol_transaction_out_106 attaches only above bip37 and asserts.

A hash resolving to no header is tested before the pruned check, as the checkpoint height query faults the store on a terminal link.

Requests remain bounded by max_inventory at deserialization, so no limit is added; an unservable item is now logged per item, where the channel previously stopped on the first.

Two cases cover the block sites. The transaction site needs the fixture to negotiate relay, which it cannot yet.

[echennells]

Closing — not ready to submit yet.