add support for async AV insertions

- add and document FI_EVENT flag for fi_av_open
- add context fields to av_insert calls
- add fields to fi_eq_entry and fi_eq_err_entry for completion of
  vectored operations
- update man pages
- implement async insrt in usnic provider

Signed-off-by: Reese Faucette <rfaucett@cisco.com>