Introduces pinned operations. When SASL bind finishes, we might still
have to maintain a link between the client an an upstream for future
bind operations if we got a SASL Bind in Progress result code. We zero
out the msgids and remember a server-unique identifer on the client and
the relevant operation that lets us retrieve that link again. This
operation is reclaimed just like anything else when connections drop.
Hopefully, this should work for LDAP TXN and VC Exop support with SASL
later as well since it allows for many-to-many links to exist.
