[Postgres Backend] password is exposed in TaskHubWorker start up log.

## Problem

When TaskHubWorker starts, it logs the postgres connection string with the password like below.

```
worker started with backend postgresql://postgres:PASSWORD@127.0.0.1:5432/durabletask
```

## Proposed solution

Postgres backend should mask password in the connection string.

```
worker started with backend postgresql://postgres:****@127.0.0.1:5432/durabletask
```