Fixes a problem with the hardcoded small timeout from PR #4725 by parameterizing the timeout in a new option -takeover_timeout_ms. It is set to a high value by default; the plan is to have -xarch_root set it to a low value for the common QEMU case of running a small test, while still overridable for large apps.
Renames -ignore_takeover_timeout to -unsafe_ignore_takeover_timeout to indicate that it can cause problems if actual application threads are left native.
Issue: #4719