add support for async AV insertions

- 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>
I would not specify a condition on when providers are free to ignore the FI_MORE flag, only that they may.