Refactor Piece to use bytes crate

[nazar-pc]

There are a few places related to pieces that can be much more efficient and convenient to use than they currently are.

Right now we have Piece(Box<PieceArray>), which means additional allocation every time we need a new piece instance.

This can be greatly optimized using https://docs.rs/bytes by having Piece(Bytes) and PieceMut(BytesMut) instead, as well as changing FlatPieces(Vec<PieceArray>) to FlatPieces(Bytes).

A few examples where this will be helpful:

• every time owned piece is more convenient to work with we can cheaply clone Piece without resorting to Arc<Piece>
• in RPC layer where we store segment of archived history, Piece can actually refer to the same memory as the segment itself, avoiding additional allocations when sending pieces via RPC