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[cpp] Allow starting lookups without blocking - #4559
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fresh-borzoni wants to merge 1 commit into
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closes #4558
A C++ lookup blocks until its result arrives, and each lookup waits for the 100 ms batch window, so one thread gets about 10 lookups per second. In Java, Rust and Python lookup() returns a future, so one thread can keep many lookups in flight.
This adds Lookuper::Lookup(pk_row, PendingLookup&) and PrefixLookuper::PrefixLookup(prefix_row, PendingPrefixLookup&), which start the lookup and return at once, the same shape as Append(row, WriteResult&). Wait(out) collects the result. Wait(out, timeout_ms) returns the retriable REQUEST_TIME_OUT if the result isn't there yet and leaves the lookup running, so it can be waited for again.
Destroying the handle before Wait returns abandons the lookup.