Apply lazy load on MySqlAuthProvider instances (#127)

Motivation:
Better load singleton instances when we really need it.

Modifications:
Apply Lazy Load on AuthProviders.

Results:
Less memory footprint.

Author: jchrys

src/main/java/io/asyncer/r2dbc/mysql/authentication/CachingSha2FastAuthProvider.java

@@ -61,7 +61,7 @@ public byte[] authentication(@Nullable CharSequence password, byte[] salt, CharC
 
     @Override
     public MySqlAuthProvider next() {
-        return CachingSha2FullAuthProvider.INSTANCE;
+        return CachingSha2FullAuthProvider.getInstance();
     }
 
     @Override

src/main/java/io/asyncer/r2dbc/mysql/authentication/CachingSha2FullAuthProvider.java

@@ -30,7 +30,9 @@
  */
 final class CachingSha2FullAuthProvider implements MySqlAuthProvider {
 
-    static final CachingSha2FullAuthProvider INSTANCE = new CachingSha2FullAuthProvider();
+    static CachingSha2FullAuthProvider getInstance() {
+        return LazyHolder.INSTANCE;
+    }
 
     @Override
     public boolean isSslNecessary() {
@@ -60,4 +62,8 @@ public String getType() {
     }
 
     private CachingSha2FullAuthProvider() { }
+
+    private static class LazyHolder {
+        private static final CachingSha2FullAuthProvider INSTANCE = new CachingSha2FullAuthProvider();
+    }
 }