[PATCH] HttpMsgPointer

Added safe HttpMsg pointer wrapper that locks and unlocks the message.

This class will not be needed if we switch to refcounting HttpMsg.
Meanwhile, it allows to pass message pointers in AsyncCalls and avoid
auto_ptr<> in exception-sensitive code.

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I am considering making the HttpMsgPointerT::lock() and unlock() methods
protected because it would be difficult for the caller to use them
correctly and because it is probably not necessary for the caller to use
them at all.

Thank you,

Alex.