Prevent potential deadlocks when database connection pool is reached

[aesteve]

Look here : https://github.com/vert-x3/vertx-jdbc-service/blob/b502a5bd1eca683b14495648d5386460a784246b/src/main/java/io/vertx/ext/jdbc/impl/JdbcServiceImpl.java#L93-L119

for the same kind of issue with pure jdbc