feature: introduce flyway as per #1059

[kezhenxu94]

Hi @nobodyiam I open this PR as per #1059 , and here is what would change in the future when we update database schemas and how users upgrade it:

1. Users run mvn -N -Pconfigdb flyway:migrate and mvn -N -Pportaldb flyway:migrate to initialize(or upgrade if needed) the database schema ApolloConfigDB and ApolloPortalDB respectively.

Users can modify the database config in scripts/db/migration/flyway-configdb.properties and scripts/db/migration/flyway-portaldb.properties

2. If we want to modify the schema, say ApolloConfigDB, add a new sql script file with versions on it, like V1.1.0__make_awesome_modification.sql under the directory scripts/db/migration/configdb/, e.g. scripts/db/migration/configdb/V1.1.0__make_awesome_modification.sql

3. Users pull the new files and re-run mvn -N -Pconfigdb flyway:migrate and mvn -N -Pportaldb flyway:migrate, all changes will be reflected on the database.

If any further changes should be done, please let me know.

Regards.

[codecov-io]

Codecov Report

Merging #1828 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1828      +/-   ##
============================================
- Coverage     48.33%   48.31%   -0.02%     
+ Complexity     1915     1913       -2     
============================================
  Files           394      394              
  Lines         11680    11680              
  Branches       1225     1225              
============================================
- Hits           5645     5643       -2     
  Misses         5589     5589              
- Partials        446      448       +2

Impacted Files	Coverage Δ	Complexity Δ
...ervice/service/ReleaseMessageServiceWithCache.java	85.71% <0%> (-1.2%)	24% <0%> (-1%)
.../apollo/internals/RemoteConfigLongPollService.java	77.3% <0%> (-0.62%)	27% <0%> (-1%)

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[coveralls]

Coverage remained the same at 52.14% when pulling 65b5031 on kezhenxu94:feature/flyway-migration into 34b912c on ctripcorp:master.

[nobodyiam]

Looks good! However I think the flyway.locations values should be corrected, as it doesn't work in my local environment.

Besides, we should also keep those legacy scripts/sql/delta sql files, since they are useful for old versions of apollo.

[kezhenxu94]

@nobodyiam thanks for reviewing, I have committed changes as requested.

[nobodyiam]

LGTM