Use modsnap for snapshot in non-legacy configurations

[morgando]

The modsnap snapshot implementation is not enabled by default right now. The changes in this PR make it the default snapshot implementation while keeping it disabled in the legacy configuration.

There were some updates to tunables to make this work:

1. The do_not_use_modsnap_for_snapshot tunable disables the modsnap implementation. This tunable is included in legacy_defaults
2. The enable_snapshot_isolation tunable now enables the modsnap implementation if modsnap was not disabled with do_not_use_modsnap_for_snapshot. If modsnap was disabled, then it enables the fallback implementation.