aio: Regularize IO worker internal naming.

aio: Regularize IO worker internal naming.

Adopt PgAioXXX convention for pgaio module type names.  Rename a
function that didn't use a pgaio_worker_ submodule prefix.  Rename the
internal submit function's arguments to match the indirectly relevant
function pointer declaration and nearby examples.  Rename the array of
handle IDs in PgAioSubmissionQueue to sqes, a term of art seen in the
systems it emulates, also clarifying that they're not IO handle
pointers as the old name might imply.

No change in behavior, just type, variable and function name cleanup.

Back-patch to 18.

Discussion: http://postgr.es/m/CA%2BhUKG%2BwbaZZ9Nwc_bTopm4f-7vDmCwLk80uKDHj9mq%2BUp0E%2Bg%40mail.gmail.com