Batch deletion of eventlogs

[ePaul]

• repository: add batch deletion method to interface (with default implementation)
• repository: implement batch deletion (uses updateBatch)
• test for batch deletion
• ~repository: refactoring for persist~
• transmission: use batch deletion

This came up as a part of #170, but seems useful enough without it, so I'm submitting this as a separate PR.

[ePaul]

:+1:

[fbrns]

👍