Timeout configuration should apply to both HTTP and gRPC

The server has configuration for controlling various timeouts, which currently only apply to HTTP requests:

```yaml
apm-server:
# Maximum amount of time to wait for the next incoming request before underlying connection is closed.
#idle_timeout: 45s

# Maximum permitted duration for reading an entire request.
#read_timeout: 30s

# Maximum permitted duration for writing a response.
#write_timeout: 30s
```

It should also be possible set timeouts for gRPC too.

I'm not convinced we need separate read/write timeouts, so let's take this as an opportunity to question whether these should be collapsed into a single request timeout.