Support `leveldb.Snapshot` in jsonrpc

[spikeekips]

Github Issue

Resolves #888

Background

See #888

Solution

• DB.OpenSnapshot method will return the snapshot id, it is the unique id of the snapshot.
• All the DB methods must have 'snapshot' in the argument.
• DB.ReleaseSnapshot method will release the snapshot of the snapshot id.
• The opened snapshot will be released automatically after expired.

The snapshot version of python example can be found at https://gist.github.com/spikeekips/a914b9747b868caabd8adc4cb53cbbe7#file-jsonrpc-sebak-prior-v0-1-2-py .

This example, https://gist.github.com/spikeekips/cc007f925435a13650f9c5a587b8e0e5 dumps and creates new leveldb.

[codecov-io]

Codecov Report

Merging #889 into master will increase coverage by 0.15%. The diff coverage is 67.6%.

@@            Coverage Diff             @@
##           master     #889      +/-   ##
==========================================
+ Coverage   60.12%   60.28%   +0.15%     
==========================================
  Files         153      154       +1     
  Lines       10839    10975     +136     
==========================================
+ Hits         6517     6616      +99     
- Misses       3558     3591      +33     
- Partials      764      768       +4

Flag	Coverage Δ
#integration_tests_long_term	44.02% <18.3%> (-0.26%)	:arrow_down:
#integration_tests_node	40% <18.3%> (-0.33%)	:arrow_down:
#unittests	48.61% <61.26%> (+0.14%)	:arrow_up:

Impacted Files	Coverage Δ
lib/errors/errors.go	0% <ø> (ø)	:arrow_up:
lib/storage/leveldb.go	58.98% <0%> (-2.91%)	:arrow_down:
lib/storage/snapshot.go	0% <0%> (ø)
lib/node/runner/jsonrpc.go	81.91% <80.67%> (-3.42%)	:arrow_down:
lib/network/validator_connection_manager.go	86.95% <0%> (+2.76%)	:arrow_up:
lib/common/message.go	42.1% <0%> (+10.52%)	:arrow_up:

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