Separate out validating v/s parsing logic in
isValidLocationConstraint() into parseLocationConstraint()
and isValidLocation()
Additionally also set `X-Amz-Bucket-Region` as part of the
common headers for the clients to fallback on in-case of any
region related errors.
This change is cleanup of the postPolicyHandler code
primarily to address the flow and also converting
certain critical parts into self contained functions.
This is a consolidation effort, avoiding usage
of naked strings in codebase. Whenever possible
use constants which can be repurposed elsewhere.
This also fixes `goconst ./...` reported issues.
