Replace downloadBlockChain() by setMode(mode)

Originally reported on Google Code with ID 98

```
Mode being one of DOWNLOAD or STANDBY.

Rationale: the concept of a "complete download of blockchain" is not meaningful in
Bitcoin, where we might be behind or on a losing fork of the chain and not know it.
We should either continuously try to download, or pause.

Suggested by andreas.schildbach in issue 90.
```

Reported by `miron@google.com` on 2011-10-28 18:53:04

Start by locating downloadBlockChain() and all of its callers, then inspect how blockchain synchronization currently represents download completion. Compare those paths with the issue's DOWNLOAD and STANDBY modes; done means the old method is replaced consistently and the affected behavior is covered by the project's relevant tests.

Written by the indexing model from the issue text.