Feat: multi-targets support

[ligangty]

• Command options "target" can accept more than one targets now
• For files uploading, will pick first target as main target to do uploading, and then copy files from main target to other targets
• For files deletion, will delete from targets one by one
• All metadadata files updating will happened sequentially
• All s3 operations will happen asynchronously

[lgtm-com]

This pull request introduces 1 alert and fixes 1 when merging 1715df5 into a7d66a4 - view on LGTM.com

new alerts:

• 1 for Syntax error

fixed alerts:

• 1 for `__eq__` not overridden when adding attributes

[coveralls]

Pull Request Test Coverage Report for Build 1921397049

• 253 of 293 (86.35%) changed or added relevant lines in 5 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.08%) to 78.213%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
charon/pkgs/pkg_utils.py	5	7	71.43%
charon/pkgs/maven.py	113	117	96.58%
charon/storage.py	60	69	86.96%
charon/cmd/command.py	2	27	7.41%

Files with Coverage Reduction	New Missed Lines	%
charon/storage.py	3	79.17%

Totals
Change from base Build 1920380743:	-0.08%
Covered Lines:	1479
Relevant Lines:	1891

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💛 - Coveralls

[ligangty]

I'll merge this first. If any problems found later then will do fix.