DAOS-15968 container: Fix CAPA fetch on NS master (#14511)

[jolivier23]

Apparently, when an I/O request fetches a container handle on the engine who is the PS leader, if the ds_cont_hdl object is absent (because the engine has restarted), then the cont_iv code won't create a ds_cont_hdl object, even though the handle is found in the RDB. This patch adds a ds_cont_tgt_open call to cont_iv_ent_fetch, so that the flow is similar to cont_iv_ent_update.

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[github-actions]

Bug-tracker data:
Ticket title is 'dfuse reported "Invalid argument" after an engine restart'
Status is 'Awaiting backport'
Labels: 'request_for_2.6.1'
https://daosio.atlassian.net/browse/DAOS-15968

[daosbuild1]

LGTM. No errors found by checkpatch.