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.

[roborivers]

Lint check:

@@ db/db_tunables.h @@
+ Please update the tunable documentation (docs/pages/config/config_files.md) accordingly.

[roborivers]

Coding style check: Error. ⚠.
Smoke testing: Success ✓.
Cbuild submission: Success ✓.
Regression testing: 318/532 tests failed ⚠.

The first 10 failing tests are:
sirandtest
sc_resume_logicalsc_generated
sc_resume
schemalk
queuedb_rollover_noroll1_generated
queuedb_rollover
timepart_trunc
selectv_rcode_serialretry_generated
selectv_rcode
selectv_rcode_disable_svonly_nop_generated

[roborivers]

Lint check:

@@ db/db_tunables.h @@
+ Please update the tunable documentation (docs/pages/config/config_files.md) accordingly.

[roborivers]

Coding style check: Error. ⚠.
Smoke testing: Success ✓.
Cbuild submission: Success ✓.
Regression testing: 12/532 tests failed ⚠.

The first 10 failing tests are:
sc_resume_logicalsc_generated
sc_timepart_logicalsc_generated
sc_resume
schemalk_logicalsc_generated
queuedb_rollover_noroll1_generated
queuedb_rollover
transchunk_manyins
sc_downgrade
truncatesc_offline_generated
comdb2sys_queueodh_generated