Reduce memory usage

[shane-kerr]

Right now the MintCoin wallet takes almost 2 GB of RAM. This is because we load the entire blockchain (and a bunch of other stuff) into memory. Increasing the amount of memory by 500 MB per year seems sub-optimal (although possibly as memory gets cheaper and bigger this will not matter eventually).

We should investigate if there are any easy ways to reduce this memory requirement. A quick look at the code shows there are probably not any easy ways, but we can look a bit more carefully.

Failing that, we should consider other options. For example we can keep only a small part of the block-chain in memory, and use the on-disk version to load as needed. We need to understand the memory access pattern to know if this makes sense, but in theory it is at least possible.