Allow pg_basebackup to stream transaction log in tar mode

This will write the received transaction log into a file called
pg_wal.tar(.gz) next to the other tarfiles instead of writing it to
base.tar. When using fetch mode, the transaction log is still written to
base.tar like before, and when used against a pre-10 server, the file
is named pg_xlog.tar.

To do this, implement a new concept of a "walmethod", which is
responsible for writing the WAL. Two implementations exist, one that
writes to a plain directory (which is also used by pg_receivexlog) and
one that writes to a tar file with optional compression.

Reviewed by Michael Paquier

src/port/tar.c

@@ -16,7 +16,7 @@
  * support only non-negative numbers, so we don't worry about the GNU rules
  * for handling negative numbers.)
  */
-static void
+void
 print_tar_number(char *s, int len, uint64 val)
 {
 	if (val < (((uint64) 1) << ((len - 1) * 3)))