Fix database name parsing when a path URL query params contains a path

[rnorth]

Postgres allows the path to certificates to be specified among the URL query params, e.g. jdbc:postgresql://host:port/dbname?sslmode=verify-full&sslrootcert=/some/path.pem. Prior to this fix, the db.instance tag would be set to path.pem instead of dbname.

This change counts slashes from the left, rather than the right, when detecting the DB name.

[coveralls]

Pull Request Test Coverage Report for Build 338

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 50.689%

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Totals
Change from base Build 337:	0.1%
Covered Lines:	515
Relevant Lines:	1016

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💛 - Coveralls