Add lz4 compression support.

LZ4 compresses data faster than gzip but at a lower ratio.  This can be a good tradeoff in certain scenarios.

Note that setting compress-type=lz4 will make new backups and archive incompatible (unrestorable) with prior versions of pgBackRest.

doc/xml/reference.xml

@@ -142,6 +142,7 @@
                         <text>The following compression types are supported:
                         <ul>
                             <li><id>gz</id> - gzip compression format</li>
+                            <li><id>lz4</id> - lz4 compression format (not available on all platforms)</li>
                         </ul></text>
 
                         <example>n</example>