[foreign-memaccess+abi] RFR: Switch to daemon threads for async upcalls

[Maurizio Cimadamore]

In a previous PR (https://git.openjdk.java.net/panama-foreign/pull/570), when merging code I forgot to re-apply the change to attach the thread as daemon thread instead of as ordinary thread.
As described previously, the daemon thread semantics is the preferred one: if the JVM waits for all background threads to stop in order to shutdown, there are cases where native libraries might not cleanup their background threads, and rely on process termination instead - which leads to a circularity (and the process never terminates).

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Commit messages:
 - Forgot to switch to daemon threads

Changes: https://git.openjdk.java.net/panama-foreign/pull/673/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=673&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/673.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/673/head:pull/673

PR: https://git.openjdk.java.net/panama-foreign/pull/673