MINOR: connection: centralize init/deinit of backend elements

MINOR: connection: centralize init/deinit of backend elements

A connection contains extra elements which are only used for the backend
side. Regroup their allocation and deallocation in two new functions
named conn_backend_init() and conn_backend_deinit().

No functional change is introduced with this commit. The new functions
are reused in place of manual alloc/dealloc in conn_new() / conn_free().
This patch will be useful for reverse connect support with connection
conversion from backend to frontend side and vice-versa.