Refactor BackEnd server to use asio-grpc

[canepat]

[codecov[bot]]

Codecov Report

Merging #793 (6301508) into master (e2728fb) will decrease coverage by 0.29%. The diff coverage is 68.93%.

@@            Coverage Diff             @@
##           master     #793      +/-   ##
==========================================
- Coverage   74.98%   74.69%   -0.30%     
==========================================
  Files         199      196       -3     
  Lines       14438    14015     -423     
  Branches     2294     2172     -122     
==========================================
- Hits        10826    10468     -358     
- Misses       2628     2735     +107     
+ Partials      984      812     -172     

Impacted Files	Coverage Δ
node/silkworm/rpc/conversion.cpp	82.35% <0.00%> (-12.80%)	:arrow_down:
node/silkworm/rpc/server/server.hpp	92.98% <0.00%> (+26.31%)	:arrow_up:
node/silkworm/rpc/server/wait_strategy.hpp	100.00% <ø> (ø)
node/silkworm/rpc/client/sentry_client.cpp	44.73% <37.50%> (-55.27%)	:arrow_down:
node/silkworm/rpc/server/server_context_pool.cpp	65.24% <42.55%> (+4.27%)	:arrow_up:
node/silkworm/rpc/server/kv_calls.cpp	72.18% <74.90%> (-6.49%)	:arrow_down:
node/silkworm/rpc/client/call.hpp	70.00% <75.00%> (+3.33%)	:arrow_up:
node/silkworm/rpc/server/backend_calls.cpp	86.85% <77.66%> (-1.07%)	:arrow_down:
node/silkworm/rpc/server/call.hpp	41.09% <86.95%> (-28.91%)	:arrow_down:
node/silkworm/backend/ethereum_backend.cpp	100.00% <100.00%> (ø)
... and 19 more

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