Determine real-time HTTP connected status

[jacogr]

Closes https://github.com/ethcore/parity/issues/3330

(Will swap to parity_ping once available)

[derhuerst]

Looks good imo. Isn't polling once a second pretty often? We already have another polling mechanism in place (HTTP HEAD /), which we might combine with this one.

[jacogr]

This is for the HTTP API, nothing to do with the UI at all. isConnected gets handled properly for the WS API (since we get feedback and make a connection, so actually there is a discrepancy between the two transports).

The UI doesn't use HTTP anywhere, while Dapps don't use the ping nor currently have feedback on the actual node. (As reported in the linked issue)

API call to be replaced with parity_ping (once available - and probably backported to beta)

[jacogr]

Merged in master for test failures.

[gavofyork]

tests still failing.

[jacogr]

Ok, looking at it yet again, these are real failures - missed a beat. Moving to in-progress until properly fixed.

[jacogr]

Failing tests finally fixed. (Expecting a sequence of mocked calls with polling timeouts in-between is not the best match for consistency)