use DBBackend set at compile time

[alessio]

Replace NewGoLevelDB() with generic NewLevelDB() calls.
DB backend is picked according to build time configuration.

Makefile:

• DB backend can be switched at compile time by
  adding WITH_CLEVELDB=yes to make install.
• Add support for user supplied ldflags and build tags.

Closes: #3719

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #3792 into develop will decrease coverage by 0.03%.
The diff coverage is 7.14%.

@@             Coverage Diff             @@
##           develop    #3792      +/-   ##
===========================================
- Coverage       61%   60.97%   -0.04%     
===========================================
  Files          191      191              
  Lines        14174    14182       +8     
===========================================
  Hits          8647     8647              
- Misses        4973     4981       +8     
  Partials       554      554

[cwgoes]

Needs PENDING.md, a question about error handling.

Also, benchmarks would be interesting.

[alexanderbez]

TestedACK. Needs a pending log entry. Also, I don't see why we should be supporting user defined ldflags? This should be only used internally by the makefile as I cannot see a circumstance where these would need to be overwritten? Do you have an example?

EDIT: Also, CI needs a kick.

[alessio]

@cwgoes @alexanderbez pending line is here

[cwgoes]

Thanks!

[alessio]

@alexanderbez dixit:

This should be only used internally by the makefile as I cannot see a circumstance where these would need to be overwritten?

They wouldn't be overwritten, but appended. Same goes for build tags.

[cwgoes]

Also, I don't see why we should be supporting user defined ldflags?

Users might want custom build options - it seems unlikely that they would define custom build flags by accident - is there a reason not to give them the option?

[cwgoes]

ACK

[alexanderbez]

@alessio @cwgoes build tags, yes, users should hav the ability to control. ldflags, not so much. Usually these tie to some internal compilation state, not sure why users would or should modify these.

[alessio]

@alexanderbez is it a blocker then? Traditionally, open source programs' build systems allow user to override {C,{,XX,PP}LD}FLAGS. I think that allowing users to customise their build does not do any harm

[cryptom-dev]

without this change does previous level db performance test result still hold...? Would it possible your previous performance test run against goleveldb...