PERL-476 support cursor in list collections

MongoDB 2.8RC3 introduces an optional cursor for listCollections; RC4 will
make it mandatory. This commit always tries to get a cursor if
available, but will fall back to the older command format for RC0 to
RC2.
PERL-476 support cursor in list indexes

MongoDB 2.8RC3 introduces an optional cursor for listIndexes; RC4 will
make it mandatory. This commit always tries to get a cursor if
available, but will fall back to the older command format for RC0 to
RC2.