lib/tdb: TDB_TRACE support (for developers)

lib/tdb: TDB_TRACE support (for developers)

When TDB_TRACE is defined (in tdb_private.h), verbose tracing of tdb operations is enabled.
This can be replayed using "replay_trace" from http://ccan.ozlabs.org/info/tdb.

The majority of this patch comes from moving internal functions to _<funcname> to
avoid double-tracing.  There should be no additional overhead for the normal (!TDB_TRACE)
case.

Note that the verbose traces compress really well with rzip.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>