Inefficient vmware eventlog downloading

There are several problems with vmware eventlog retrieval logic:

1. Eventlogs are always downloaded in batches of 10, even if there are thousands of old records (0.5sp).
2. If poller did not check collected event logs between collector data refreshes (so it did not set service.eventlog_last_key to the lastlogsize value), the cached events will be discarded and collector will redownload them (1-2sp).

The first can be solved by dynamically increasing the batch size with each next request. Start with 10 and multiply by 2 (until batch size reaches 1000) if ReadPreviousEvents returned valid events.

The second must be investigated more carefully - it seems to be the case, but needs to be tested.