Optional database encryption support on database creation.

[xq262144]

Add several bugfixes including:

1. Optional database encryption support on database creation.
2. SQLChain layer block production race condition bugfix.
3. Client connection id allocation and error handling.
4. Deadly simple runtime space resource limit checker.
5. Merged new command line tool for SQL query & database creation/destroy.

[leventeliu]

LGTM.

[covenantdb]

[golint] _{reported by reviewdog 🐶}
if block ends with a return statement, so drop this else and outdent its block (move short variable declaration to its own line if necessary)

[auxten]

nice!

[codecov]

Codecov Report

Merging #60 into develop will increase coverage by 0.35%.
The diff coverage is 58%.

@@             Coverage Diff             @@
##           develop      #60      +/-   ##
===========================================
+ Coverage    76.24%   76.59%   +0.35%     
===========================================
  Files          126      126              
  Lines        10117    10192      +75     
===========================================
+ Hits          7714     7807      +93     
+ Misses        1516     1508       -8     
+ Partials       887      877      -10

[covenantdb]

[golint] _{reported by reviewdog 🐶}
if block ends with a return statement, so drop this else and outdent its block (move short variable declaration to its own line if necessary)