Body downloader structure

[mriccobene]

This PR introduces changes in the structure of the header downloader and adds the structure of the body downloader.

The following diagram depicts the overall architecture:

The old header downloader is splitted in two classes:

• HeadersStage: it governs the header downloading process and save headers on the db using the read-write tx received from the stage-loop; it has forward() and unwind_to() methods
• BlockDownloader: it receive messages (new headers, requested headers, request for headers, ...) from remote peers and messages from the HeadersStage; it process messages (using the "command pattern") acting on HeaderChain

In the same manner the body downloader is divided in two classes:

• BodiesStage: it governs the body downloading process and save bodies on the db using the read-write tx received from the stage-loop; it has forward() and unwind_to() methods
• BlockDownloader: it receive messages (new bodies, requested bodies, request for bodies, ...) from remote peers and messages from the BodiesStage and acts on BodySequence

Two classes have the responsibility to implement headers and body downloading algorithms: HeaderChain and BodySequence. They are members of BlockDownloader whose responsibility is to receive messages and process them. Incoming messages (from remote peers) carry informations to HeaderChain and BodySequence (for example: hash announcemens, headers, bodies, ...). Also outgoing messages request information to HeaderChain and BodySequence (for example: new header or bodies needed) and send it to remote peers. Internal messages are a way for the objects in different thread to communicate.

Threading model:

• stage classes run in the same thread of the stage-loop
• BlockDownloader runs in its own thread and uses message-passing as communication mechanism with the other threads
• SentryClient runs in its own thread

[codecov]

Codecov Report

Merging #630 (71a24a4) into master (f2a7a59) will decrease coverage by 0.02%.
The diff coverage is 63.79%.

@@            Coverage Diff             @@
##           master     #630      +/-   ##
==========================================
- Coverage   82.16%   82.13%   -0.03%     
==========================================
  Files         173      173              
  Lines       14425    14424       -1     
==========================================
- Hits        11852    11847       -5     
- Misses       2573     2577       +4     

Impacted Files	Coverage Δ
...ilkworm/downloader/packets/block_bodies_packet.hpp	0.00% <ø> (ø)
...lkworm/downloader/packets/block_headers_packet.hpp	0.00% <ø> (ø)
...orm/downloader/packets/get_block_bodies_packet.hpp	0.00% <ø> (ø)
...rm/downloader/packets/get_block_headers_packet.hpp	0.00% <ø> (ø)
...ode/silkworm/downloader/packets/hash_or_number.hpp	63.15% <ø> (ø)
...orm/downloader/packets/new_block_hashes_packet.hpp	0.00% <ø> (ø)
...e/silkworm/downloader/packets/new_block_packet.hpp	0.00% <ø> (ø)
node/silkworm/downloader/packets/rlp_decoding.cpp	64.21% <ø> (ø)
node/silkworm/downloader/packets/rlp_encoding.cpp	96.87% <ø> (ø)
...orm/downloader/packets/rlp_eth66_packet_coding.hpp	80.48% <ø> (ø)
... and 3 more

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