Users don't expect a new reference returned from a get() function.
Indeed, all users of that API I foud where NOT destroying the reference.
Just change the implementations to NOT return a reference.

This applies to the following APIs:
hb_unicode_funcs_get_default()
hb_glib_get_unicode_funcs()
hb_icu_get_unicode_funcs()

Fixes https://github.com/harfbuzz/harfbuzz/issues/1134

Fixes https://github.com/harfbuzz/harfbuzz/issues/1129

We've had the proper barriers already.  No need for more with every access.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1127

This reverts commit 66920a6b.

Some of our bots (Oracle Studio and Apple gcc 4.2) do not allow
explicit except for constructors.

https://github.com/harfbuzz/harfbuzz/issues/1127

Was creating object even if wasn't there.

Found and fixed a couple bugs.

Found a couple multithreading issues.  Marked them with "XXX-MT-bug".

Removes volatile from fallback implementation.  That was handwavy
anyway.

If ever int is not 32bit, we'll fix this...

https://bugzilla.mozilla.org/show_bug.cgi?id=1476334