vinyl: lookup key in reader thread

If a key isn't found in the tuple cache, we fetch it from a run file. In
this case disk read and page decompression is done by a reader thread,
however key lookup in the fetched page is still performed by the tx
thread. Since pages are immutable, this could as well be done by the
reader thread, which would allow us to save some precious CPU cycles for
tx.

Close #4257