Start block parameter skipped when a watcher is started first time

[akolotov]

Even if the parameters HOME_START_BLOCK and FOREIGN_START_BLOCK are configured in .env file they will not be used when the bridge is run first time since they are strings where as the code expects them to be integers:

https://github.com/poanetwork/bridge-nodejs/blob/8359556d9e9976a9388720d97de70e1bd7b35af8/src/watcher.js#L27

https://github.com/poanetwork/bridge-nodejs/blob/8359556d9e9976a9388720d97de70e1bd7b35af8/src/watcher.js#L106-L112

Scenario to reproduce:

1. Deploy the bridge contracts in the erc-to-erc mode. Store bridgeDeploymentResults.json created after the deployment.
2. Wait for few blocks produced
3. Transfer tokens from Foreign network
4. Wait for few blocks produced
5. Configure .env of the bridge instance and points HOME_START_BLOCK and FOREIGN_START_BLOCK with values inserted in bridgeDeploymentResults.json after the contracts deployment.
6. Run the bridge processes with empty Redis DB.
   As soon as the watcher-erc-affirmation-request process started following appeared in the terminal:

"Found 0 Transfer events"

and the corresponding requests to relay assets (transfer calls) does not handled by the bridge.

[akolotov]

Addressed in #103