DSN optimizations (part 4)

[nazar-pc]

This is where refactoring was going up until this point.

During plotting:

• farmer will only announce pieces that were not plotted (and thus announced before)
• pieces that satisfy caching policy will be cached and announced right away
• cached pieces are returned from cache without reaching to the network at all

During announcements:

• pieces that were previously announced are ignored
• announced pieces are retrieved from announcer only (instead of making full network retrieval with lookup)

There is still one more optimization I came up with, but left it for later in #1100

Fixes #1076

Code contributor checklist:

• I have read, understood and followed contributing guide

[nazar-pc]

@i1i1 please look carefully at the description of this PR, it describes some key changes that were partially true on the SDK side, but not entirely (with node and farmer you'll have two caches: recently archived segments on node and regular piece cache on the farmer for instance). It'll need to be corrected and I'll do further refactoring to expose more of the primitives that you can assemble the way you need it in SDK (right now there will be a bit more copy-paste required than I'd like to see).

[shamil-gadelshin]

LGTM