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conn: expose the OK/EOF packet warning count (#2)

* conn: expose the OK/EOF packet warning count

MySQL reports how many warnings a statement raised in the packet that
terminates its response, but the driver dropped the field on the floor: a
literal `// warning count [2 bytes]` comment in handleOkPacket with no
parse behind it, and three copies of the resultset terminator reader that
skipped straight past it to the status flags.

Without the count, a caller that wants to surface warnings has only bad
options. The diagnostics themselves live in per-connection state that
only SHOW WARNINGS can read, so reading them costs a round trip; with no
count to gate on, that round trip has to be spent after every statement,
warnings or not. The count is what makes it affordable — and it is also
what a proxy needs to put in its own OK packet, since clients such as
Connector/J only ask for warnings when the count they were given is
non-zero.

Record it on the connection alongside the status flags, clear it in
clearResult() so one statement's count cannot be read as the next one's,
and expose it as (*mysqlConn).Warnings() for callers reaching the
connection through (*sql.Conn).Raw.

The three inlined resultset terminator readers collapse into one
readResultsetTerminator method. They were already identical; the warning
count made keeping them identical harder, because an EOF packet orders
warnings before status and an 0xFE-headered OK packet orders them after,
so each copy would have had to get that backwards independently.

* review: correct the warning-count lifecycle docs

Both comments on the PR were right, and both were about wording that
described an earlier draft rather than the code.

handleOkPacket no longer has a statusMoreResultsExists early return, so
"recorded before the more-results check" described a check that is not
there. Say what the code now does and why: the count is recorded
unconditionally so each statement of a multi-statement leaves its own
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