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 think we only want to define this at AV creation.  Otherwise, the provider needs to handle a mix of blocking and asynchronous insertion calls, which may be messy.