Streams: items compression implemented.

The approach used is to set a fixed header at the start of every
listpack blob (that contains many entries). The header contains a
"master" ID and fields, that are initially just obtained from the first
entry inserted in the listpack, so that the first enty is always well
compressed. Later every new entry is checked against these fields, and
if it matches, the SAMEFIELD flag is set in the entry so that we know to
just use the master entry flags. The IDs are always delta-encoded
against the first entry. This approach avoids cascading effects in which
entries are encoded depending on the previous entries, in order to avoid
complexity and rewritings of the data when data is removed in the middle
(which is a planned feature).

src/stream.h

@@ -26,6 +26,11 @@ typedef struct stream {
  * rewriting code that also needs to iterate the stream to emit the XADD
  * commands. */
 typedef struct streamIterator {
+    streamID master_id;     /* ID of the master entry at listpack head. */
+    uint64_t master_fields_count;       /* Master entries # of fields. */
+    unsigned char *master_fields_start; /* Master entries start in listapck. */
+    unsigned char *master_fields_ptr;   /* Master field to emit next. */
+    int entry_flags;                    /* Flags of entry we are emitting. */
     uint64_t start_key[2];  /* Start key as 128 bit big endian. */
     uint64_t end_key[2];    /* End key as 128 bit big endian. */
     raxIterator ri;         /* Rax iterator. */