maxPoolSize and maxThreadSize should be configurable

[draganpodvezanec-verox]

After applying PR
#1427

...we can establish a much larger number of HTTP connections to the local Geoserver pool, and seed many layers in parallel. But there is a limitation on max seeder threads, which is hardcoded:

<!-- Thread pool for seeding -->
  <bean id="gwcSeederThreadPoolExec" 
    class="org.geowebcache.seed.SeederThreadPoolExecutor">
    <constructor-arg value="16"/><!-- Size of core pool -->
    <constructor-arg value="32"/><!-- Maximum size of pool -->
  </bean>

This should be possible to configure using the environment variable.