Lend stored extents to reads instead of copying them

A read of a stored document now answers with the store mapping's extents
through writev, without copying the document. pure_draft_mapping takes a
reference to the mapping a draft opened through a handle reads from; the
answer keeps it until the last byte is sent, so the version read goes out
intact even if the document changes or a compaction replaces the file
meanwhile. Replies the mapping does not hold are copied as before.

pure-serve's output is now a list of extents (the head first). One thread
serves 2,761 1 MiB reads per second (axum: 1,380) and 128k small ones.

0 files changed, 0 insertions, 0 deletions