A static VarHandle should inline better than AtomicInteger, which
requires calling through potentially megamorphic calls for several
atomic operations.

This also avoids allocating AtomicObject.

Most runs of JRuby will never use coverage data, so allocating this
eagerly is wasted time and memory most of the time. This patch
makes Ruby.coverageData allocation lazy, only if directly
requested, and adds a no-allocation way to test if coverage has
been enabled. This may improve performance of execution paths that
previously acquired the field, invoked "isCoverageEnabled", read
another field, and compared that result with a constant. these
paths are potentially very hot since coverage events typically
happen on every line.