Improve memory usage

The current client takes more than 600 MB of memory. We must reduce that.

A first quick investigation confirmed that it's the block index that takes almost all the memory. We added a lot of vote data to it, and especially some std::maps and vectors that take a significant amount of memory, even when they're empty.

Another problem is the startup time, that seems to be slowed down due to the index loading.

Notice that the slow loading does not always happen and probably unrelated with the memory usage.

I made a first proposal in pull request #246. There's a lot more we can do, but it involves more code changes.

About startup time, yes it's mostly the loading of the block index (when the files are in cache loading is very quick). We could try to reduce the size of the disk block index.

One possibility to reduce memory usage would be to only keep the most recent vote data. To do that we could store them independently from the block index, and load them on demand and cache them (because we may still need old data occasionally).

A more radical solution was proposed in pull request #253. It can work in addition to pull request #246.

When will this problem be solved with the votes? The wallet is already using at least 830 MB and sometimes it even uses >900 MB. I've to choose between keeping my wallet open or browsing. Doing both will crash my laptop.

The situation will be improved in version 2.1.0.