This commit includes changes to the server to ensure that the utilities:
pg_rewind and pg_basebackup can be changed to support recovery in a
multi-segment-singular-host setting. We link pg_tblspc to a <dbid>
subdirectory of the tablespace, rather than to the path of the
tablespace directly, and we remove the <dbid> from the tablespace
version directory. At the same time, we have designed towards preserving
the response to pg_tablespace_location(<tablespace_oid>) such that it
does not return the dbid suffix. The design is such that it is the
responsibility of the utilities to append the dbid as and when required.

Before this commit:
  * the symlink to the tablespace directory looks like:
      pg_tblspc/spcoid/ -> /<tablespace_location>/
  * Under the symlink target, we would have the following:
    GPDB_MAJORVER_CATVER_db<dbid>/dboid/relfilenode
  * pg_tablespace_location(tsoid) returns: <tablespace_location>
e.g.
  * pg_tblspc/20981/ -> /data1/tsp1
  * Under /data1/tsp1: GPDB_6_201902061_db1/19849/192814
  * pg_tablespace_location(20981) returns: /data1/tsp1

After this commit:
  * the symlink to the tablespace directory looks like:
      pg_tblspc/spcoid/ -> /<tablespace_location>/<dbid>
  * Under the symlink target, we would have the following:
      GPDB_MAJORVER_CATVER/dboid/relfilenode
  * pg_tablespace_location(tsoid) returns: <tablespace_location>

e.g.
  * pg_tblspc/20981/ -> /data1/tsp1/1
  * Under /data1/tsp1/1: GPDB_6_201902061/19849/192814
  * pg_tablespace_location(20981) returns: /data1/tsp1

Motivation:

When tablespaces were aligned to upstream postgres, while removing
filespaces, we added the `tablespace_version_directory()` function to
supply each segment with a unique tablespace directory name. This was
accomplished by appending the 'magic' `GpIdentity.dbid` global variable
to the `GP_TABLESPACE_VERSION_DIRECTORY` in `tablespace_version_directory()`.

This is problematic for several reasons- but perhaps most severely is
the fact that in order to use any code in libpgcommon.so that references
this value, you need to first set the `GpIdentity.dbid` global,
otherwise any functions that deal with tablespaces will be broken in
unpredictable ways.

An example is pg_rewind- where `GetRelationPath()` will not return a valid
relation unless you repeatedly toggle the `GpIdentity.dbid` between the
value of the source or target segment dependant on the context of which
relfiles are being examined.

This commit bumps the catalog version here we have made breaking changes
in the tablespace filesystem layout.
Co-authored-by: NAdam Berlin <aberlin@pivotal.io>
Co-authored-by: NTaylor Vesely <tvesely@pivotal.io>
Co-authored-by: NSoumyadeep Chakraborty <sochakraborty@pivotal.io>