Make HttpNamedEndpoint keep shared pointer to connection

This makes it possible to avoid segmentation faults in the case where the remote end drops the connection.
Previously it would lead to the connection object being deleted and a dangling pointer.

Mostly relevant to long-running connections that are handled asynchronously.  In the synchronous case the
connection is locked for the duration and recycled afterwards, so it can't cause a problem.
Merge pull request #105 from datacratic/PLAT-570-HttpNamedEndpoint-keep-connections-3

Make HttpNamedEndpoint keep shared pointer to connection (PLAT-570)