multi: do transfer book keeping using mid

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

lib/Makefile.inc

@@ -236,6 +236,9 @@ LIB_CFILES =         \
   timediff.c         \
   timeval.c          \
   transfer.c         \
+  uint-bset.c        \
+  uint-spbset.c      \
+  uint-table.c       \
   url.c              \
   urlapi.c           \
   version.c          \
@@ -376,6 +379,9 @@ LIB_HFILES =         \
   timediff.h         \
   timeval.h          \
   transfer.h         \
+  uint-bset.h        \
+  uint-spbset.h      \
+  uint-table.h       \
   url.h              \
   urlapi-int.h       \
   urldata.h          \