* sync: keep pushed ciphers as UTF-8 from the hub to MongoDB
Pushed ciphers were parsed into strings, then encoded back to UTF-8 by
the driver into a new buffer for each large one: the two largest
sources of large object heap allocations on prod.
PushItems now reads each cipher straight into a pooled UTF-8 buffer and
UpsertMany writes the item's BSON itself, from that buffer, into another
pooled one. The stored documents are unchanged. Validation moves to
SyncItemValidator, and the base64 check is vectorized.
* sync: hand back pushed buffers when a push is rejected
The hub disposed the pushed items only after its own checks, and the
authorization filter rejects a call after SignalR has bound it, so those
rejections left their pooled buffers to the GC. A push with null items
also made the cleanup throw over the real error.
Abdullah Atta