OPTIM: backend: set release on takeover for strict maxconn

OPTIM: backend: set release on takeover for strict maxconn

When strict maxconn is enforced on a server, it may be necessary to kill
an idle connection to never exceed the limit. To be able to delete a
connection from any thread, takeover is first used to migrate it on the
current thread prior to its deletion.

As takeover is performed to delete a connection instead of reusing it,
<release> argument can be set to true. This removes unnecessary
allocations of resources prior to connection deletion. As such, this
patch is a small optimization for strict maxconn implementation.

Note that this patch depends on the previous one which removes any
assumption in takeover implementation that thread isolation is active if
<release> is true.