Monitor::wait takes signed integer as timeout

In `mutex.cpp`, `Monitor::wait(int64_t timeout, ...)` takes a signed integer as timeout, and asserts it must be positive , `assert(timeout >= 0, "negative timeout");`. Changing it to `uint64_t` could forbid invalid values.

A contrived example to trigger the assertion above using the `fastdebug` build: `java -XX:+UseZGC -Xms1g -XX:SoftMaxHeapSize=2g -Xmx4g -XX:ZUncommitDelay=9223372036854775 --version`.