Allow overriding AWS S3 Region with env var

[harlow]

Previously the S3 connection was initialized as the global variable before env.Parse() was called.

By moving the initialization into the main func we can leverage the env helper functions.

• Add s3 connection to blockCache
• Initialize s3 connection after env has been parsed

Addresses:

region = "us-east-1" // TODO(kr): figure out how to not hard code this

From commit: https://github.com/chain/chain/commit/aca1da719fc31f34df9a96c20ef3715a9e3493d4

[CLAassistant]

All committers have signed the CLA.

[harlow]

cc @kr lemme know your thoughts