Fix LDAP pool parameter setting

LDAP pool is activated by context property com.sun.jndi.ldap.connect.pool in LDAPDirectory, which is correct. However the pool configuration should be system properties, not context properties as they are done today.

The Configuration doc is pretty clear about that.

Also the wiki and extension point doc should be updated to talk about the server poolingEnabled attribute (see LDAPServerDescriptor).

https://stackoverflow.com/questions/22411967/which-ldap-jndi-provider-pool-settings-are-system-properties-and-which-are-envi has a clearer description of what is context (environment) property and what is system property.