Remove unnecessary mutexes in RPC server

[canepat]

This PR removes unnecessary synchronization code:

• mutexes and scoped lockes in rpc::Server startup/shutdown and
• shutdown thread in BackEnd and KV server main

Such a refactoring is possible because the BackEndKV server threading model changed since the synchronization introduction at the beginning of the implementation.

Specifically, each rpc::ServerContext now executes on its own threaded scheduler powered by asio::io_context, as opposed to a couple of threads per context in the first implementation. This simplifies the shutdown sequence which can be now started by the asio::io_context intercepting the termination signals and carried on by each rpc::ServerContext for its data structures (namely, grpc::CompletionQueue objects).

[codecov]

Codecov Report

Merging #646 (a773873) into master (536cde7) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
+ Coverage   82.53%   82.58%   +0.04%     
==========================================
  Files         176      176              
  Lines       15239    15236       -3     
==========================================
+ Hits        12578    12583       +5     
+ Misses       2661     2653       -8     

Impacted Files	Coverage Δ
node/silkworm/rpc/server_context_pool.cpp	93.75% <ø> (-0.13%)	⬇️
node/silkworm/rpc/server_context_pool.hpp	100.00% <ø> (+16.66%)	⬆️
node/silkworm/rpc/server.hpp	97.53% <100.00%> (-0.04%)	⬇️
node/silkworm/rpc/call.hpp	93.73% <0.00%> (+0.50%)	⬆️
core/silkworm/state/intra_block_state.cpp	97.46% <0.00%> (+0.94%)	⬆️
core/silkworm/state/in_memory_state.cpp	95.12% <0.00%> (+0.97%)	⬆️

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