Add zstream redup command to convert deduplicated send streams

Deduplicated send and receive is deprecated.  To ease migration to the
new dedup-send-less world, the commit adds a `zstream redup` utility to
convert deduplicated send streams to normal streams, so that they can
continue to be received indefinitely.

The new `zstream` command also replaces the functionality of
`zstreamdump`, by way of the `zstream dump` subcommand.  The
`zstreamdump` command is replaced by a shell script which invokes
`zstream dump`.

The way that `zstream redup` works under the hood is that as we read the
send stream, we build up a hash table which maps from `<GUID, object,
offset> -> <file_offset>`.

Whenever we see a WRITE record, we add a new entry to the hash table,
which indicates where in the stream file to find the WRITE record for
this block. (The key is `drr_toguid, drr_object, drr_offset`.)

For entries other than WRITE_BYREF, we pass them through unchanged
(except for the running checksum, which is recalculated).

For WRITE_BYREF records, we change them to WRITE records.  We find the
referenced WRITE record by looking in the hash table (for the record
with key `drr_refguid, drr_refobject, drr_refoffset`), and then reading
the record header and payload from the specified offset in the stream
file.  This is why the stream can not be a pipe.  The found WRITE record
replaces the WRITE_BYREF record, with its `drr_toguid`, `drr_object`,
and `drr_offset` fields changed to be the same as the WRITE_BYREF's
(i.e. we are writing the same logical block, but with the data supplied
by the previous WRITE record).

This algorithm requires memory proportional to the number of WRITE
records (same as `zfs send -D`), but the size per WRITE record is
relatively low (40 bytes, vs. 72 for `zfs send -D`).  A 1TB send stream
with 8KB blocks (`recordsize=8k`) would use around 5GB of RAM to
"redup".

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10124 
Closes #10156

lib/libzfs/libzfs_sendrecv.c

@@ -61,6 +61,7 @@
 #include "zfs_prop.h"
 #include "zfs_fletcher.h"
 #include "libzfs_impl.h"
+#include <cityhash.h>
 #include <zlib.h>
 #include <sys/zio_checksum.h>
 #include <sys/dsl_crypt.h>
@@ -5518,9 +5519,7 @@ zfs_receive_impl(libzfs_handle_t *hdl, const char *tosnap,
 	}
 
 	/* Holds feature is set once in the compound stream header. */
-	boolean_t holds = (DMU_GET_FEATUREFLAGS(drrb->drr_versioninfo) &
-	    DMU_BACKUP_FEATURE_HOLDS);
-	if (holds)
+	if (featureflags & DMU_BACKUP_FEATURE_HOLDS)
 		flags->holds = B_TRUE;
 
 	if (strchr(drrb->drr_toname, '@') == NULL) {