Use constants for worker table column names

[JatinSanghvi]

• Closes Replace string literals with constants for worker table column names #308

[JatinSanghvi]

@Charles-Gagnon, we can rename column names to say _az_func_AttemptCount, etc. to make conflicts extremely unlikely. This would be a breaking change (where worker table is already created with older schema) but hopefully that will be acceptable by the team dependent on it at this point.

Also, please let me know if you have a different take on how to replace string literals with constants.

There are columns in global state table too that can be converted to constants.

[JatinSanghvi]

@Charles-Gagnon, if we are going to change the table schema, I would also like to change the table name from Worker_* to Leases_* as the table is specific to user function, not to a worker (considering a worker is a function app instance).

[JatinSanghvi]

@lucyzhang929, @Charles-Gagnon, I will issue another PR to rename the worker table to leases table. Cosmos DB extension uses similar name as default name for the collection that is used to maintain leases (link).