[WIP] Refactoring of event listeners(closes #8, #89, #123, #297)

[Koc]

Code is not tested yet, WIP.

1. do not call recompute changeset because changesets not yet computed when preFlush triggers
2. fixes #8, #89, #123, #297
3. register only one event listener per db driver instead of registering one event listener for each mapping
4. reduce calls for reading metadata

Looks like AdapterInterface::recomputeChangeSet is not called anymore.

[Koc]

@stof what can you say about this PR in general? And I have some problem with Propel integration. Not sure this approach will work correctly

[garak]

@Koc can you solve conflict? And maybe adding some tests?

[Koc]

@garak yes, will do it later after fiinish my other PRs. Also I should apply changes requested by @stof . This PR is much harder than other.

[Koc]

branch restarted and rebased. I'm afraid that this PR breaks BC. But we can start working on 2.0 where BC-breaks are acceptable.