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kv: Introduce MuxRangeFeed bi-directional RPC.
As demonstrated in cockroachdb#74219, running rangefeed over large enough table or perhaps not consuming rangefeed events quickly enough, may cause undesirable memory blow up, including up to OOMing the node. Since rangefeeds tend to run on (almost) every node, a possiblity exists that a entire cluster might get distabilized. One of the reasons for this is that the memory used by low(er) level http2 RPC transport is not currently being tracked -- nor do we have a mechanism to add such tracking. cockroachdb#74456 provided partial mitigation for this issue, where a single RangeFeed stream could be restricted to use less memory. Nonetheless, the possibility of excessive memory usage remains since the number of rangefeed streams in the system could be very large. This PR introduce a new bi-directional streaming RPC called `MuxRangeFeed`. In a uni-directonal streaming RPC, the client estabilishes connection to the KV server and requests to receive events for 1 span. A separate RPC stream is created for each span. In contrast, `MuxRangeFeed` is bi-directional RPC: the client is expected to connect to the kv server and request as many spans as it wishes to receive from that server. The server multiplexes all events onto a single stream, and the client de-multiplexes those events appropriately on the receiving end. Note: just like in a uni-directional implementation, each call to `MuxRangeFeed` method (i.e. a logical rangefeed established by the client) is treated independently. That is, even though it is possible to multiplex all logical rangefeed streams onto a single bi-directional stream to a single KV node, this optimization is not currently implementation as it raises questions about scheduling and fairness. If we need to further reduce the number of bi-directional streams in the future, from `number_logical_feeds*number_of_nodes` down to the `number_of_nodes`, such optimization can be added at a later time. Multiplexing all of the spans hosted on a node onto a single bi-directional stream reduces the number of such stream from the `number_of_ranges` down to `number_of_nodes` (per logical range feed). This, in turn, enables the client to reserve the worst case amount of memory before starting the rangefeed. This amount of memory is bound by the number of nodes in the cluster times per-stream maximum memory -- default is `2MB`, but this can be changed via dedicated rangefeed connection class. The server side of the equation may also benefit from knowing how many rangefeeds are running right now. Even though the server is somewhat protected from memory blow up via http2 pushback, we may want to also manage server side memory better. Memory accounting for client (and possible server) will be added in the follow on PRs. The new RPC is turned on by default since the intention is to retire the uni-directional rangefeed during later release. However, this default maybe disable (and rangefeed reverted to use old implementation) by setting `COCKROACH_ENABLE_MULTIPLEXING_RANGEFEED` environment variable to `false` and restarting the cluster. Release Notes (enterprise change) Rangefeeds (which are used internally e.g. by changefeeds) now use a common HTTP/2 stream per client for all range replicas on a node, instead of one per replica. This significantly reduces the amount of network buffer memory usage, which could cause nodes to run out of memory if a client was slow to consume events. The rangefeed implementation can be revered back to its old implementation via restarting the cluster with the `COCKROACH_ENABLE_MULTIPLEXING_RANGEFEED` environment variable set to false.
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