avatica-go should handle complex/long URLs

The current avatica-go driver assumes the full URL path is the schema name. This is incorrect for some deployments - for example, using a proxy.

Current behavior:

dsn = "http://host.com/service/proxy/schema"

conf.schema = "service/proxy/schema"

Expected behavior:

dsn = "http://host.com/service/proxy/schema"

conf.schema = "schema"