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

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.