Exploit unplotted plot space as extra caching space

[nazar-pc]

Many farmers are creating smaller farms (1T or less) and with default cache size of just 1% most of the pieces are being downloaded from the network.

We can improve this dramatically by filling up the space that wasn't plotted yet with cache of pieces and storing the pieces that were previously downloaded in there instead of requesting them from the network.

It will be a bit messy implementation-wise with things crossing areas of responsibility and I don't think we want to do that for regular piece cache, but it will improve bandwidth usage dramatically and as such is likely worth doing to improve user experience.

Related to #2398

[nazar-pc]

This can be implemented as a separate complementary caching layer that would transparently parasite on top of plots for as long as possible in addition to regular caching mechanism.