RFR: 8372344: QUIC transfer may deadlock if pacing rate is too high

[Daniel Jeliński]

Modify the PacketTransmissionTask to update the current deadline (`now`) between loop iterations.

This fixes the deadlock issue which happened when the next deadline was equal to the current one, but no progress could be made because the pacer quota only increases as time passes.

No new test; the issue was extremely rare. Existing tests continue to pass.

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Commit messages:
 - Fix deadlock in packet transmission task

Changes: https://git.openjdk.org/jdk/pull/28462/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28462&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372344
  Stats: 10 lines in 1 file changed: 2 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/28462.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28462/head:pull/28462

PR: https://git.openjdk.org/jdk/pull/28462