h2 block store download sometimes stops

Originally reported on Google Code with ID 409

```
hello ,
sometimes , h2 stops downloading blockchain and i never get message like this ;
"AbstractBlockChain add"
i dont know the reason , i did not see any exception on log maybe for internet connection
lose temporary may got this problem.

It should continue download , retry.
it maybe the problem i said or not.

When i restart application , it continues to download blockchains.
You should fix this issue...

```

Reported by `kadir.bayner` on 2013-05-21 15:54:26

Start by tracing the H2 block store download path around the AbstractBlockChain add point mentioned in the report. Reproduce a temporary internet interruption and compare behavior with restarting the application. Done means block download retries and resumes without requiring an application restart, without silently stopping.

Written by the indexing model from the issue text.