channelstorage_cpp_map_name_id: Add read locking around retrievals.

channelstorage_cpp_map_name_id: Add read locking around retrievals.

When we retrieve a channel from a C++ map, we actually get back a wrapper
object that points to the channel then right after we retrieve it, we bump its
reference count.  There's a tiny chance however that between those two
statements a delete and/or unref might happen which would cause the wrapper
object or the channel itself to become invalid resulting in a SEGV.  To avoid
this we now perform a read lock on the driver around those statements.

Resolves: #1491