Possibility to set a JDBC connection  as the EJB TimerPool resource

Add possibility to set a JDBC connection pool using the glassfish __TimerPool.

It's needed if you need to use EJB Timer services on a distributed environment and you want to share the timer database (Or just using an other vendor in place of Derby...).

Example of the POM.xml if using a MySQL database
```
...
<jdbcResource>
    <jndiName>jdbc/custom-timer</jndiName>
    <user>MyUserName</user>
    <password>CrazyPassword</password>
    <url>jdbc:mysql://localhost:3306/TIMER</url>
     <!-- This connection will use the timer pool -->
    <timerPool>true</timerPool>
</jdbcResource>
...
```

The database needs to be created (Example for MySQL) before using the time service.

```
CREATE TABLE `TIMER`.`EJB__TIMER__TBL` (
    `CREATIONTIMERAW`      BIGINT        NOT NULL,
    `BLOB`                 BLOB,
    `TIMERID`              VARCHAR(255)  NOT NULL,
    `CONTAINERID`          BIGINT        NOT NULL,
    `OWNERID`              VARCHAR(255)  NULL,
    `STATE`                INTEGER       NOT NULL,
    `PKHASHCODE`           INTEGER       NOT NULL,
    `INTERVALDURATION`     BIGINT        NOT NULL,
    `INITIALEXPIRATIONRAW` BIGINT        NOT NULL,
    `LASTEXPIRATIONRAW`    BIGINT        NOT NULL,
    `SCHEDULE`             VARCHAR(255)  NULL,
    `APPLICATIONID`        BIGINT        NOT NULL,
    CONSTRAINT `PK_EJB__TIMER__TBL` PRIMARY KEY (`TIMERID`) 
);
```

This change is retro-compatible and doesn't impact previous version of the Maven plugin.

Enjoy,
Is this setter required ? For the other parameter values, we do not define any setter at the moment.