bug(configtxlator): nil pointer dereference in computeUpdt and missing graceful shutdown

There is a logic error in `configtxlator/main.go` within the `computeUpdt` function that can cause a nil pointer dereference panic under certain conditions. In addition, the REST server lacks proper lifecycle management.

## Details
### Nil Pointer Dereference
Around lines 226-228, the code assigns `cu.ChannelId = channelID` before performing the `if cu == nil` guard. If `update.Compute()` returns `nil`, this assignment causes the program to panic with a nil pointer dereference instead of returning the intended error message. As a result, the nil check on line 228 is effectively dead code.

### Missing Graceful Shutdown
The REST server initialized by `startServer()` lacks graceful shutdown mechanisms, signal handling, and context-based cancellation. The server currently blocks indefinitely on `http.Serve`.

## Expected Behavior
- If `update.Compute()` returns `nil`, the program should handle it safely and return the appropriate error message without panicking.
- The `configtxlator` REST server should handle termination signals properly and shut down gracefully without indefinitely blocking.

## Actual Behavior
- The application crashes with a nil pointer dereference panic.
- The server process blocks indefinitely and cannot be cleanly terminated.

## Proposed Solution
- Reorder the `cu == nil` check to occur before the `cu.ChannelId` field assignment.
- Implement signal-based graceful shutdown with context cancellation for the REST server.
- Introduce a `/healthz` endpoint for the server.
- Add unit tests covering the `computeUpdt` edge cases.