Extend DBDRIVER to load from json_file or json_http and add EvalOnlyMode

[zhouzhuojie]

Description

Introduced two new sources of evaluation cache.

• json_file
• json_http

We can leverage FLAGR_DB_DBDRIVER and FLAGR_DB_DBCONNECTIONSTR and make flagr instances into EvalOnlyMode, which means it will only serve traffic for /api/v1/evaluation/*. It's the first attempt to separate Flagr into data plane and control plane. And EvalOnlyMode makes Flagr even easier to deploy with evaluation cache decoupled from the DB.

/**                                                                                  
DBDriver and DBConnectionStr define how we can write and read flags data.            
For databases, flagr supports sqlite3, mysql and postgres.                           
For read-only evaluation, flagr supports file and http.                              
                                                                                     
Examples:                                                                            
                                                                                     
FLAGR_DB_DBDRIVER     FLAGR_DB_DBCONNECTIONSTR                                       
=================     ===============================================================
"sqlite3"             "/tmp/file.db"                                                 
"sqlite3"             ":memory:"                                                     
"mysql"               "root:@tcp(127.0.0.1:18100)/flagr?parseTime=true"              
"postgres"            "host=myhost user=root dbname=flagr password=mypassword"       
                                                                                     
"json_file"           "/tmp/flags.json"                    # (sets EvalOnlyMode=true)
"json_http"           "https://example.com/flags.json"     # (sets EvalOnlyMode=true)
                                                                                     
*/                                                                                   

This PR also introduced

• /api/v1/export/eval_cache/json endpoint
• Adding back SnapshotID to flag's JSON serialization (because we need it to be included in the export, and thus json_file and json_http can load it as well)
• Refactor eval_cache and adding refresh timeout support

How Has This Been Tested?

Tested locally.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[ziru]

lgtm

[codecov-io]

Codecov Report

Merging #247 into master will decrease coverage by 0.19%.
The diff coverage is 81.37%.

@@            Coverage Diff            @@
##           master     #247     +/-   ##
=========================================
- Coverage   82.47%   82.27%   -0.2%     
=========================================
  Files          25       26      +1     
  Lines        1444     1518     +74     
=========================================
+ Hits         1191     1249     +58     
- Misses        193      202      +9     
- Partials       60       67      +7

Impacted Files	Coverage Δ
pkg/entity/flag.go	94.28% <ø> (ø)	⬆️
pkg/handler/fixture.go	100% <100%> (ø)	⬆️
pkg/handler/export.go	69.09% <100%> (+2.42%)	⬆️
pkg/handler/handler.go	85.41% <42.85%> (-7.61%)	⬇️
pkg/config/config.go	83.33% <50%> (-2.39%)	⬇️
pkg/handler/eval_cache_fetcher.go	82.35% <82.35%> (ø)
pkg/handler/eval_cache.go	75% <94.11%> (ø)	⬆️

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