Implement dynamodb based request and metric stores

[cfkanesan]

• implemented dynamodb backend for request and metric stores
• float type is not supported by dynamodb, values are cast to and from Decimal type, precision is not important because it's mainly timestamps
• the request table has secondary indexes on the user and status fields to enable querying (constant lookup time)
• the metrics table has a secondary index on the request_id
• the index definitions are motivated by access patterns observed elsewhere in the codebase
• user_id field is duplicated at the top level to use as the hash key for a global secondary index for the request table
• request_id field on the metrics table is dropped if NULL, this makes the index sparse
• the user index query takes precedence on the status index query since the former is more restrictive
• the wipe functions are not implemented because the testing is done in units against moto and are thus not needed
• the endpoint url config can be used to run against a local instance of dynamodb
• collect_metric_info is not yet implemented, dynamodb reports metrics directly to Cloudwatch, could be needed if another monitoring system is used

[FussyDuck]

All committers have signed the CLA.

[peshence]

Looks good to me

[sametd]

Looks good.