sstable compression (#1946)

* compress sstable with zstd

* add some details to sstable readme

* compress only block which benefit from it

* multiple changes to sstable

make compression optional
use OwnedBytes instead of impl Read in sstable, required for next point
use zstd bulk api, which is much faster on small records

* cleanup and use bulk api for compression

* use dedicated byte for compression

* switch block len and compression flag

* change default zstd level in sstable

ownedbytes/src/lib.rs

@@ -139,6 +139,16 @@ impl OwnedBytes {
         self.advance(8);
         u64::from_le_bytes(octlet)
     }
+
+    /// Reads an `u32` encoded as little-endian from the `OwnedBytes` and advance by 4 bytes.
+    #[inline]
+    pub fn read_u32(&mut self) -> u32 {
+        assert!(self.len() > 3);
+
+        let quad: [u8; 4] = self.as_slice()[..4].try_into().unwrap();
+        self.advance(4);
+        u32::from_le_bytes(quad)
+    }
 }
 
 impl fmt::Debug for OwnedBytes {