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

tests/data/Makefile.am

@@ -276,6 +276,6 @@ test3032 \
 test3100 test3101 test3102 test3103 test3104 test3105 \
 \
 test3200 test3201 test3202 test3203 test3204 test3205 test3207 test3208 \
-test3209 test3210
+test3209 test3210 test3211 test3212 test3213
 
 EXTRA_DIST = $(TESTCASES) DISABLED

tests/data/test3211

@@ -0,0 +1,22 @@
+<testcase>
+<info>
+<keywords>
+unittest
+uint_bset
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<server>
+none
+</server>
+<features>
+unittest
+</features>
+<name>
+uint_bset unit tests
+</name>
+</client>
+</testcase>

tests/data/test3212

@@ -0,0 +1,22 @@
+<testcase>
+<info>
+<keywords>
+unittest
+uint_bset
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<server>
+none
+</server>
+<features>
+unittest
+</features>
+<name>
+uint_bset unit tests
+</name>
+</client>
+</testcase>

tests/data/test3213

@@ -0,0 +1,22 @@
+<testcase>
+<info>
+<keywords>
+unittest
+uint_spbset
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<server>
+none
+</server>
+<features>
+unittest
+</features>
+<name>
+uint_spbset unit tests
+</name>
+</client>
+</testcase>