Index entities on the correct Solr Environment

At our company we use this module quite extensively but we faced one issue related to the fact that the module always assumes the default apachesolr environment through this call:

$env_id = apachesolr_default_environment();

In projects like ours, where we have different environments per entity type, we lose the benefits of the module and it actually gets a bit confusing, since it also indexes every time on the default solr env_id given the above call.

Brainstorming a little bit, the ideal scenario would be that module fetches the correct solr enviromnents ids (could be more than one) per entity type and then prepare doc and call apachesolr_realtime_commit ...

Attaching a patch to this module that fixes the issue above described for entity insert/update and delete