localbuf: Introduce StartLocalBufferIO()

localbuf: Introduce StartLocalBufferIO()

To initiate IO on a shared buffer we have StartBufferIO(). For temporary table
buffers no similar function exists - likely because the code for that
currently is very simple due to the lack of concurrency.

However, the upcoming AIO support will make it possible to re-encounter a
local buffer, while the buffer already is the target of IO. In that case we
need to wait for already in-progress IO to complete. This commit makes it
easier to add the necessary code, by introducing StartLocalBufferIO().

Reviewed-by: Melanie Plageman 
Discussion: https://postgr.es/m/CAAKRu_b9anbWzEs5AAF9WCvcEVmgz-1AkHSQ-CLLy-p7WHzvFw@mail.gmail.com