[WIP] Attempt EZP-22932: Add commit type configuration to Solr handler

Change the commit type used by solr handler to "soft" by default as hard commits in Solr 4.x seems to be handled automatically by autoCommit by default. Expose configuration so this is configurable.

Todo
- [ ] Don't expose setCommitType(), only "setCommitConfigurationEnabled()", otherwise inline hardcoding of type is encouraged while this should rather be a indexing strategy for configuration
- [ ] Discuss the needs for commit config, what needs to be exposed in a Solr 4.x env (some older options are not so relevant anymore, and hence should be skipped)
- [ ] Wire up yml config for this, in a site access/group/repo aware way
- [ ] Get feedback on other things to improve regarding indexing.