BUG/MINOR: backend: fix improper insert in avail tree for always reuse

BUG/MINOR: backend: fix improper insert in avail tree for always reuse

In connect_server(), if http-reuse always is set, the backend connection
is inserted into the available tree as soon as created. However, the
hash connection field is only set later at the end of the function.

This seems to have no impact as the hash connection field is always
position before a lookup. However, this is not a proper usage of ebmb
API. Fix this by setting the hash connection field before the insertion
into the avail tree.

This must be backported up to 2.4.