Retry S3 RequestTimeTooSkewed errors instead of immediately terminating.

Fix incorrect handling of transfer-encoding response to HEAD request.

Fix scoping violations exposed by optimizations in gcc 9.

Add repo-s3-port option for setting a non-standard S3 service port.

The local command for backup is implemented entirely in C.

The check command is implemented partly in C.

Add Perl interface to C storage layer.

Add Db object to encapsulate queries and commands.

Add PostgreSQL query client.