bigfile: ZODB -> BigFileH invalidate propagation
Continuing theme from the previous patch, here is propagation of invalidation messages from ZODB to BigFileH memory. The use-case here is that e.g. one fileh mapping was created in one connection, another in another, and after doing changes in second connection and committing there, the first fileh has to invalidate appropriate already-loaded pages, so its next transaction won't work with stale data. To do it, we hook into ZBlk._p_invalidate() and propagate the invalidation message to ZBigFile which then notifies all opened-through-it ZBigFileH to invalidate a page. ZBlk -> ZBigFile lookup is done without storing backpointer in ZODB - instead, every time ZBigFile touches ZBlk object (and thus potentially does GHOST -> Live transition to it), we (re-)bind it back to ZBigFile. Since ZBigFile is the only class that works with ZBlk objects it is safe to do so. For ZBigFile to notify "all-opened-through-it" ZBigFileH, a weakset is introduced to track them. Otherwise the real page invalidation work is done by virtmem (see previous patch).
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