fix: fromSeq stack-safe

[Prikaz98]

I encountered a StackOverflowError while trying to get an AsyncStream from a long sequence using fromSeq, along with filter and take operations.

[info]   java.lang.StackOverflowError:
[info]   at com.twitter.concurrent.AsyncStream.$anonfun$filter$2(AsyncStream.scala:250)
[info]   at com.twitter.util.Return.map(Try.scala:308)
[info]   at com.twitter.util.Future.$anonfun$map$1(Future.scala:1939)
[info]   at com.twitter.util.ConstFuture.transformTry(ConstFuture.scala:47)
[info]   at com.twitter.util.Future.map(Future.scala:1939)
[info]   at com.twitter.concurrent.AsyncStream.filter(AsyncStream.scala:249)
[info]   at com.twitter.concurrent.AsyncStream.$anonfun$filter$4(AsyncStream.scala:253)
[info]   at com.twitter.util.Return.map(Try.scala:308)
[info]   at com.twitter.util.Future.$anonfun$map$1(Future.scala:1939)

I wrote a unit test that reproduces this behavior.

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