See 69448b13a1 / #1969; the compiler can now ensure that a uint64
intended for atomic access is always aligned to a 64-bit boundary.
Convert atomic operations on uint32s and pointers as well.

On a 32-bit architecture, 64-bit atomic loads and stores must be aligned to a
64-bit boundary. Since the (mysql.MySQL) struct is directly included in the
Server struct, it is impossible to guarantee this via the standard technique
of putting the 64-bit value at the beginning of the struct definition
(since the point at which it is included in the parent struct may cross a
64-bit boundary).

This optimization is probably pointless anyway, adding an additional
indirection won't make a difference.

It was sending MESSAGE_ERROR, which was inappropriate. Send an empty batch
for now (this is at parity with the in-memory implementation).

This reverts commit 5bbee02fe6.

Fix various pagination issues with CHATHISTORY; also undo #491
(msgid munging).

Allow users to set max MySQL connections and connection lifetime;
set a sane default for max connections if it's not present.

Remove the CHATHISTORY * and znc.in/playback *self targets,
clean up associated database code, add new mechanisms to play
all missed DMs.

Implements the new `CHATHISTORY LISTCORRESPONDENTS` API.

This adds a new configuration option, socket-path, instead of using
host.