add benchmark for chunker

We get about 300MB/s - so this is still not fast enough

Note: This is quite slow with debug target, so please compile with --release

src/bin/test_chunk_speed.rs

@@ -0,0 +1,43 @@
+extern crate proxmox_backup;
+
+use proxmox_backup::backup::chunker::*;
+
+fn main() {
+
+    let mut buffer = Vec::new();
+
+    for i in 0..1024*1024 {
+        for j in 0..4 {
+            let byte = ((i >> (j<<3))&0xff) as u8;
+            //println!("BYTE {}", byte);
+            buffer.push(byte);
+        }
+    }
+    let mut chunker = Chunker::new(512*1024);
+
+    let count = 100;
+
+    let start = std::time::SystemTime::now();
+    
+    for _i in 0..count {
+        let mut pos = 0;
+        while pos < buffer.len() {
+            let k = chunker.scan(&buffer[pos..]);
+            if k == 0 {
+                //println!("LAST {}", pos);
+                break;
+            } else {
+                pos += k;
+                //println!("CHUNK {}", pos);
+            }
+        }
+    }
+
+    let elapsed = start.elapsed().unwrap();
+    let elapsed = (elapsed.as_secs() as f64) +
+        (elapsed.subsec_millis() as f64)/1000.0;
+    
+    let mbytecount = ((count*buffer.len()) as f64) / (1024.0*1024.0);
+    let mbytes_per_sec =  mbytecount/elapsed; 
+    println!("SPEED = {} MB/s", mbytes_per_sec); 
+}