zcash_client_backend: Switch from protobuf 2 to prost 0.11

[str4d]

The latter is maintained by the Tokio developers, and has easy integration with the tonic gRPC library which is actively maintained.

[codecov]

Codecov Report

Base: 73.64% // Head: 73.67% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (68e2149) compared to base (df1cbbd).
Patch coverage: 27.27% of modified lines in pull request are covered.

❗ Current head 68e2149 differs from pull request most recent head 587e1fa. Consider uploading reports for the commit 587e1fa to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #691      +/-   ##
==========================================
+ Coverage   73.64%   73.67%   +0.03%     
==========================================
  Files         104      104              
  Lines       10157    10155       -2     
==========================================
+ Hits         7480     7482       +2     
+ Misses       2677     2673       -4     

Impacted Files	Coverage Δ
zcash_client_backend/src/data_api/error.rs	16.66% <0.00%> (ø)
zcash_client_backend/src/welding_rig.rs	97.36% <ø> (ø)
zcash_client_sqlite/src/chain.rs	20.77% <20.00%> (ø)
zcash_client_backend/src/proto.rs	80.00% <40.00%> (+4.32%)	⬆️
zcash_primitives/src/sapling/redjubjub.rs	90.32% <0.00%> (-1.08%)	⬇️
zcash_client_backend/src/scan.rs	41.66% <0.00%> (+1.19%)	⬆️
zcash_history/src/entry.rs	29.16% <0.00%> (+2.08%)	⬆️

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[nuttycom]

utACK modulo Clippy complaints.