[tunnelbroker] Fix MESSAGES_TABLE_NAME constant

Summary:
Was getting the following:

tunnelbroker-server    | terminate called after throwing an instance of 'std::runtime_error'
tunnelbroker-server    |   what():  Error: AWS DynamoDB table 'tunnelbroker-message' is not available

Checked the AWS console and it looks like the messages table name is tunnelbroker-messages instead of tunnelbroker-message?

Update:

Following our code convention about database names, fix dynamoDB table names to plural.

Linear task: ENG-1091

Landing procedure:

After landing, the running service in the AWS cloud needs to be restarted/updated using the newly built docker image.
The old tables must be removed from the AWS DynamoDB before the new container will be restarted.

Test Plan:
No longer getting the above error.
Run yarn run-tunnelbroker-service and use new table names.

Reviewers: varun, karol-bisztyga, atul, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D3213