Use WALReadFromBuffers in more places

Hi,

Commit 91f2cae7a4e that introduced WALReadFromBuffers only used it for
physical walsenders. It can also be used in more places benefitting
logical walsenders, backends running pg_walinspect and logical
decoding functions if the WAL is available in WAL buffers. I'm
attaching a 0001 patch for this.

While at it, I've also added a test module in 0002 patch to
demonstrate 2 things: 1) how the caller can ensure the requested WAL
is fully copied to WAL buffers using WaitXLogInsertionsToFinish before
reading from WAL buffers. 2) how one can implement an xlogreader
page_read callback to read unflushed/not-yet-flushed WAL directly from
WAL buffers. FWIW, a separate test module to explicitly test the new
function is suggested here -
https://www.postgresql.org/message-id/CAFiTN-sE7CJn-ZFj%2B-0Wv6TNytv_fp4n%2BeCszspxJ3mt77t5ig%40mail.gmail.com.

Please have a look at the attached patches.

I will register this for the next commit fest.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com