This adds auto-expire support for the DynamoDB backend, via the DynamoDB
Time to Live feature.

boto3 version 1.9.178 requires botocore>=1.12.178.

botocore version 1.12.178 introduces support for the DynamoDB
UpdateTimeToLive call.

The UpdateTimeToLive call is used by the DynamoDB backend to enable TTL
support on a newly created table.

Handle TTL enabling/disabling separately from the table get-or-create
function.

Improve handling of cases where the TTL is already set to the desired
state.

DynamoDB only allows a single TTL update action within a fairly long
time window, so some problematic cases (changing the TTL attribute,
enabling/disabling TTL when it was recently modified) will raise
exceptions that have to be dealt with.

If the boto3 TTL methods are not found, log an informative error. If the
user wants to enable TTL, raise an exception; if TTL should be disabled,
simply return.

- Handle exceptions by logging the error and re-raising

- Log (level debug) when the desired TTL state is already in place

Additional changes:

- Handle exceptions when calling boto3's describe_time_to_live()

- Fix test cases for missing TTL methods