Move WAL-related definitions from dbcommands.h to separate header file.

Heikki Linnakangas <heikki.linnakangas@iki.fi>

Commit: f1fd515b393a283d7368ce45a92709d2c7f9a991
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: 2015-03-09T13:50:49Z
Releases: 9.5.0
Move WAL-related definitions from dbcommands.h to separate header file.

This makes it easier to write frontend programs that needs to understand
the WAL record format of CREATE/DROP DATABASE. dbcommands.h cannot easily
be #included in a frontend program, because it pulls in other header files
that need backend stuff, but the new dbcommands_xlog.h header file has
fewer dependencies.

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