multi: process pending, one by one

multi: process pending, one by one

Before curl 8.14.0, when pending was a list, `process_pending_handles()`
move a single transfer to processing. In 8.14.0 we changed that to move
all pending transfers to processing. This lead to unwanted performance
drops as reported in #18017.

Restore the old behaviour.

While the old behviour is better, the overall handling of "pending"
transfers is not optimal, since we do not keep track of the "condition"
a pending transfer is waiting on. This means, when moving a single,
pending transfer, we might move one that still cannot be processed while
another that could is kept pending.

Since we trigger `process_pending_handles()` from various changes, the
stalled pending will eventually make it to the processing queue, but
this is not optimal.

Fixes #18017
Reported-by: rm-rmonaghan on github
Closes #18056