fix(cmd/paratime): Fix proposer stats

[peternose]

Ensure that a node is not counted twice as a proposer when it serves the roles of both a worker and a backup worker.

Testnet:

./oasis paratime statistics 17993756 17993757

Before:

|                  ENTITY ADDR                   | PRIMARY | BACKUP | PROPOSER |
|------------------------------------------------|---------|--------|----------|
| oasis1qphtg2256r8q3ljs6j5yqg0em8t62ycv9q7yysls |       1 |      1 |        2 |

After:

|                  ENTITY ADDR                   | PRIMARY | BACKUP | PROPOSER |
|------------------------------------------------|---------|--------|----------|
| oasis1qphtg2256r8q3ljs6j5yqg0em8t62ycv9q7yysls |       1 |      1 |        1 |