Allow maximum request timeout to be larger than write timeout (avoid EOF)

[peterbroadhurst]

The WriteTimeout in Go is actually the time all the way from the reading of the headers, until the data has been fully written in the response.

In an example E2E test with confirmations, so that synchronous calls take longer than 15s to complete, we see:

Websocket 127.0.0.1:5000 event: default/token_pool_confirmed/339d3f53-37de-4baa-bd46-e4277aeb315e -> c4a76e57-d586-4cbf-9581-b0deb5352ec4 (tx=%!s(*core.Transaction=<nil>))
    restclient.go:552: 
                Error Trace:    /Users/pbroadhurst/dev/firefly/firefly/test/e2e/runners/restclient.go:552
                                                        /Users/pbroadhurst/dev/firefly/firefly/test/e2e/runners/tokens.go:231
                Error:          Received unexpected error:
                                Post "http://127.0.0.1:5000/api/v1/namespaces/default/tokens/pools?confirm=true": EOF
                Test:           TestEthereumMultipartyE2ESuite/TestE2ENonFungibleTokensSync

This PR allows the constructor of the HTTP Server to include a MaximumRequestTimeout, which forces the read timeout and the write timeout (not the read header timeout) to be larger than this value.

It adds 1 second buffer, so that a nice error message can be returned in the case of a context based request timeout.