Output the following in pg_xlogdump for prepare transaction records:
* Global transaction ID
* Prepare time
* Tablespace OIDs:
	* 'tablespace_oid_to_delete_on_abort'
	* 'tablespace_oid_to_delete_on_commit'

These data come from TwoPhaseFileHeader structure which is written in
xlog in GPDB.

Move TwoPhaseFileHeader to `twophase.h` in order for that structure to
be used by `xact_desc_prepare()` in `xactdesc.c`
Co-Authored-By: NDavid Kimura <dkimura@pivotal.io>