Clear RequestContext when spawning connections

[ReubenBond]

This was causing reliability issues because new connections could occasionally be started due to a Ping message, which has a "Ping" RequestContext header set. The connection would then capture that RequestContext and would import it into messages created by the SiloConnection itself. These messages would then be treated as though they were Ping messages by the remote silo.

Examples of such messages are:

• Rejections for messages destined to an old silo generation
• Directory lookups - this is trickier, it relies on an optimization whereby IncomingMessageAgent.ReceiveMessage is called synchronously from the connection

This would result in timeouts (which can prevent a silo from starting), active silos being declared dead, obscure exceptions such as Unrecoverable Rejection (info: System.InvalidOperationException: Expected result of type Orleans.GrainDirectory.AddressesAndTag but encountered a null value.), and NullReferenceExceptions from ActivationDirectory.FindTarget

[ReubenBond]

Merging myself after approval - we've already tested this in VSO