read stream on amcheck

Hi,

While reviewing some other patches implementing stream API for core subsystems,
I noticed that the amcheck extension could also benefit from that.

Notice the refactor when handling the "skip" parameter; The logic was moved to
the heapam_read_stream_next_block callback so that verify_heapam don't need to
touch any private field of heapam_read_stream_next_block_private struct.

One other think to mention is that the test cases of "skip" parameter
that I've seen just test when the first page is corrupted, so I think
that a carefully review on callback logic would be good to ensure that
we don't accidentally skip a page when doing p->current_blocknum++;

This patch doesn't show any performance improvements (or regression)
but I think that it would be good to replace the ReadBufferExtended
usage with the read stream API, so in the future it could be benefit
from the AIO project.

--
Matheus Alcantara