Proposer Should Add Block to Cache When Building the Block

Otherwise, if we have only notarized a block and then try and build/verify the next block we will not be able to fetch it's parent.

Some logs from an instance test that was flaking
```
[09-01|10:45:02.507] DEBUG msm/msm.go:370 Error while deciding whether to build a block {"test": "TestInstance_OfflineDuringTransition", "myNodeID": 3, "error": "context canceled"}
[09-01|10:45:02.507] DEBUG simplex/epoch.go:2847 Not starting monitoring for block notarization because we were cancelled while waiting for block to be built {"test": "TestInstance_OfflineDuringTransition", "myNodeID": 3, "epoch round": 2, "monitored round": 2}
[09-01|10:45:02.507] DEBUG simplex/monitor.go:89 Task executed {"test": "TestInstance_OfflineDuringTransition", "myNodeID": 3, "taskID": 30}
[09-01|10:45:02.507] DEBUG simplex/monitor.go:87 Executing f {"test": "TestInstance_OfflineDuringTransition", "myNodeID": 3, "taskID": 31}
[09-01|10:45:02.507] DEBUG msm/msm.go:303 WaitForPendingBlock failed to get block {"test": "TestInstance_OfflineDuringTransition", "myNodeID": 3, "seq": 2, "error": "block not found"}
```