Small fixes in exonum-merkledb

[alekseysidorov]

• Restore snapshot method in ServiceContext.

Definition of Done

• There are no TODOs left in the merged code
• Change is covered by automated tests
• The coding guidelines are followed
• Public API has proper documentation
• Changelog is updated if needed (in case of notable or breaking changes)
• The continuous integration build passes

[codecov]

Codecov Report

Merging #1303 into master will increase coverage by <.01%.
The diff coverage is 94.5%.

@@            Coverage Diff             @@
##           master    #1303      +/-   ##
==========================================
+ Coverage   93.18%   93.18%   +<.01%     
==========================================
  Files         162      162              
  Lines       24949    24899      -50     
==========================================
- Hits        23248    23202      -46     
+ Misses       1701     1697       -4

Impacted Files	Coverage Δ
exonum/src/blockchain/service.rs	72.97% <ø> (ø)	⬆️
components/merkledb/src/proof_map_index/proof.rs	94.56% <ø> (ø)	⬆️
exonum/src/helpers/fabric/maintenance.rs	27.02% <0%> (+0.71%)	⬆️
testkit/tests/counter/counter.rs	98.76% <100%> (ø)	⬆️
...sts/inflating_currency/inflating_cryptocurrency.rs	95.37% <100%> (ø)	⬆️
services/time/tests/test_exonum_time.rs	94.73% <100%> (-0.31%)	⬇️
services/configuration/src/schema.rs	96.8% <100%> (ø)	⬆️
examples/timestamping/backend/src/schema.rs	96.66% <100%> (ø)	⬆️
services/time/src/schema.rs	91.66% <100%> (ø)	⬆️
components/merkledb/src/db.rs	92.64% <100%> (ø)	⬆️
... and 8 more

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