For "timeline ancestor merge" or "timeline detach," we need to "cut"
delta layers at particular LSN. The name "truncate" is not used as it
would imply that a layer file changes, instead of what happens: we copy
keys with Lsn less than a "cut point".
Cc: #6994
Add the "copy delta layer prefix" operation to DeltaLayerInner, re-using
some of the vectored read internals. The code is `cfg(test)` until it
will be used later with a more complete integration test.
8d0f701767