Change multi's book keeping of transfers to no longer use lists, but a
special table and bitsets for unsigned int values.
`multi-xfers` is the `uint_tbl` where `multi_add_handle()` inserts a new
transfer which assigns it a unique identifier `mid`. Use bitsets to keep
track of transfers that are in state "process" or "pending" or
"msgsent".
Use sparse bitsets to replace `conn->easyq` and event handlings tracking
of transfers per socket. Instead of pointers, keep the mids involved.
Provide base data structures and document them in docs/internal:
* `uint_tbl`: a table of transfers with `mid` as lookup key,
handing out a mid for adds between 0 - capacity.
* `uint_bset`: a bitset keeping unsigned ints from 0 - capacity.
* `uint_spbset`: a sparse bitset for keeping a small number of
unsigned int values
* `uint_hash`: for associating `mid`s with a pointer.
This makes the `mid` the recommended way to refer to transfers inside
the same multi without risk of running into a UAF.
Modifying table and bitsets is safe while iterating over them. Overall
memory requirements are lower as with the double linked list apprach.
Closes#16761
