Add synchronization to IndefiniteStatementCache.cache

The access to the backing Map of IndefiniteStatementCache is now synchronized.

[resolves #232]

Author: mp911de

src/main/java/io/r2dbc/postgresql/IndefiniteStatementCache.java

@@ -46,8 +46,10 @@ public Mono<String> getName(Binding binding, String sql) {
         Assert.requireNonNull(binding, "binding must not be null");
         Assert.requireNonNull(sql, "sql must not be null");
 
-        return this.cache.computeIfAbsent(Tuples.of(sql, binding.getParameterTypes()),
-            tuple -> this.parse(tuple.getT1(), tuple.getT2()));
+        synchronized (this.cache) {
+            return this.cache.computeIfAbsent(Tuples.of(sql, binding.getParameterTypes()),
+                tuple -> this.parse(tuple.getT1(), tuple.getT2()));
+        }
     }
 
     @Override