MySQL/MariaDB default to a "utf8" character set that may only
encode the BMP (max 3 bytes). In MySQL/MariaDB 5.5 and later
there is an additional character set "utf8mb4" that also supports
the code points outside the BMP. This new character set however
requires redefining tables, etc for it to be able to be used.

As a work-around we instead default to keep using the "utf8"
character set while encoding characters outside the BMP with
surrogate pairs. This works seemlessly with old table definitions,
while having the minor defect of characters outside the BMP not
collating as single characters.

Fixes [PIKE-112].

sizeof(Thread.Queue()) is not equal to Thread.Queue()->size().
Fixes the calculation for adaptive buffer refills.
Without this fix, it *will* result in hung queries, but only
if the number of rows returned exceeds 1024.

- Solves the last remaining one-deadlock-per-month problem.
- Speeds up the critical path with regard to stash-flushing.