Fix SQL parsing for MVT provider

[johngian]

• Make ID field optional
• Fix geometry sniffing for MVT queries
• Fix handling MVT provider SQL when using (SELECT ...) as alias queries

[ARolek]

@johngian it looks like this PR is failing the test suite: https://github.com/go-spatial/tegola/pull/744/checks?check_run_id=2015097575. I can help look into this if that would be helpful.

[coveralls]

Pull Request Test Coverage Report for Build cbedc67ec-PR-744

• 35 of 42 (83.33%) changed or added relevant lines in 3 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.02%) to 45.673%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
provider/postgis/postgis.go	13	16	81.25%
provider/postgis/util.go	16	20	80.0%
<!--	Total:	35	42	83.33%	-->

Files with Coverage Reduction	New Missed Lines	%
server/viewer.go	3	61.54%
server/server.go	4	83.33%
<!--	Total:	7		-->

Totals
Change from base Build 8e40cafbb:	0.02%
Covered Lines:	5409
Relevant Lines:	11843

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💛 - Coveralls