Allocate all bufferpool shards of the same size, regardless of
underlying allocator. This way increase memory usage a bit, but we're
already quite heavy on that, so no big deal.
Change-Id: Icbe09cd2f9b33bc58ab1efe7de293dea00236fec
Revert of 128e84e and implement for io_uring poller
Turns out that this function is necessary to prevent out-of-order writes
to a streambuf once the connection is open.
Use a simple array to keep track of blocked fds under uring.
Change-Id: I4af2a64071030fd4892dde88547705230aec59fd