[MERGE][1.6][#254797] Fix NotImplementedError when probing for smart protocol via HTTP.

[Andrew Bennetts]

This fixes a regression introduced in 1.6.  Probing for a smart server over HTTP was
triggering a NotImplementedError, because read_line was not implemented on the
HTTP medium request class.

This fix implements that method, and does some very simple refactoring to do so
without duplicating code.

It also adds a simple test to test_http that fails without this fix.
Implementing that test required improving the HTTPServerWithSmarts test helper
to behave a little more realistically.

Despite the size of the diff, this is a simple and I believe safe change to
incorporate in 1.6 final.  The bulk of the diff affects code that was not
working without this fix, or tests for that fix.  The rest is a straightforward
refactoring of code that has extensive test coverage.

-Andrew.

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