tsgrpc_kvstore driver - how to signal `generation_if_equal` mismatch on write?

Hi,

I'm trying to implement a gRPC service for the tsgrpc_kvstore driver, and trying to understand how to correctly support the `generation_if_equal` option for `WriteRequest`. In particular, how to handle the case where the generation does not match - the comment in the proto says "the write is aborted" on a mismatch, what exactly does this mean?

Both returning an error from the RPC and returning an error in the `WriteResponse.status` field leads to the TensorStore write to fail immediately, which leads me to believe this is not what I should do?

Looking at [PerformWriteback](https://github.com/google/tensorstore/blob/933712f0628f9170ed00eafdd561402eb6fb77fe/tensorstore/kvstore/transaction.cc#L134) I see that the transaction is re-tried if the returned generation is "unknown" (== empty). However if I try this, TensorStore just sends writes with `generation_if_equal=StorageGeneration::NoValue()` in a loop, instead of what I would expect:
1. Attempt a write with an old generation, receive StorageGeneration::Unknown
2. Issue a read to obtain the current value of the file
3. Issue a new write, with the update generation and value

thank you!

PS: I'm also a bit unclear what I should use for the `timestamp` value of `GenerationAndTimestamp` - is this intended to be the current time of the gRPC server? The last modified time of the file?