refactor: move ApiServer&ApiHandler to cpp file and overwrite config …

[chenbxun]

…after creating snapshot per review

What this PR does / why we need it:
Make the following adjustment according to the code review:

• move the definition of ApiServer and ApiHandler from api_server.h to api_server.cpp
• overwrite the config file in use when creating snapshot in case the old one is used again after the process restarts
• use safe_delete instead of delete

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?