Reduce xlog.h inclusion footprint

Reduce xlog.h inclusion footprint

This file needs xlogreader.h only for the XLogReaderState typedef; but
we can dodge that by forward-declaring it.  Many files use xlog.h for
reasons other than reading WAL, and it's not good to force all those
files to include xlogreader.h, so take it out.

Surprisingly, there is no fallout in core code from making this change.
Perhaps external code will have to start including xlogreader.h.