RFR: 8242038: G1: Lazily initialize RSHashTables

[Kim Barrett]

> On Apr 7, 2020, at 3:57 AM, Kim Barrett <kim.barrett at oracle.com> wrote:
> Morover, the RSHashTable constructor calls RSHashTable::clear(). That
> calls SparsePRTEntry::size(), whose value is computed from
> G1RSetSparseRegionEntries, which is a product flag which doesn't have
> its final value at static initialization time. Oops!

Oh, and SparsePRTEntry::size() is also called to size the array.

Probably the thing that makes this not just crash quickly is that the special
empty_table doesn’t seem to get used as anything other than a marker / sentinal.