Session hijacking is a design mistake.
Without it Proxy has full ownership of the connections.
This also simplifies the overall workflow and makes code less error-prone.

Fixes #497

Modifiers are changed to return errors.

Unify error handling for all callers of writeResponse().

Reduce the scope of writeResponse() and unify proxyConn and proxyHandler response writing API.

Unify error logging in proxyConn and proxyHandler.

The returned modifier error is debug logged and error response is created.
The error response is passed to ResponseModifier, if that fails error is error logged.
The error response is sent back to client as normal response.

Fixes #498
Fixes #525

Remove Session as it has no purpose beyond hijacking.
Move secure connection detection to newProxyConn(),
thanks to that we no longer call locking ConnectionState() on every request.

ErrorStatus enables communication between martian packages and ErrorResponse() implementations.
ErrorStatus can be returned as a request or response modification error and instructs ErrorResponse() what is the desired status code.

As we handle RequestModifier errors now it makes sense to fail fast.
The RequestModifier responsibility has been moved to martian.ErrorStatus, and will be handled in Forwarder.

The use RT skipping is limited to testing only, as on RequestModifier error the RT is automatically skipped.
Replace RT skipping per context with global Proxy.TestingSkipRoundTrip for testing purposes.

Add traceID type responsible for the request trace ID generation.
The traceID is attached to request context on read.
Remove Context struct as it serves no purpose.

…ntext keys

Remove all logic from logger the package just map calls from package level function to the provided Logger impl.
Add context to the Logger interface, this allows to add custom logger wrappers using the context information.

Move trace ID logging logic close to traceID.
Use logger stacking to add features to logs.

This is only temporary and traceID should be moved to the toplevel log package.
To make it work we would need to change the main Logger interface, see #544.

Treat label "-" as ignore indicator, and do not record deny errors in errors metric.

Fixes #498