R1:3dfa89dd5d27

Update SES API to use AWSv4 signatures

Summary:
Ref T13570. Fixes T13235. In most cases, we use modern (v4) signatures for almost all AWS API calls, and have for several years.

However, sending email via SES currently uses an older piece of external code which uses the older (v3) signature method.

AWS is retiring v3 signatures on October 1 2020, so this pathway will stop working.

Update the pathway to use `PhutilAWSFuture`, which provides v4 signatures.

T13235 discusses poor error messages from SES.