Massively refactor and sanify condition collapsing

Wow... what a ride. This commit adds a faithful reimplementation of the
SQLA descend algorithm, but instead of SQL produces a reduced HASH that
nevertheless corresponds to the original query 1:1 (or so I *really* hope)

This is another one of these "I can try it with DQ but I don't even have
tests" stories, so once again opted to implement things the "dumb" way.

The benefits are quite substantial:
- Better deduplication of WHERE condition
- Consolidated functions - the vaguely similar logic in the current version
  of _collapse_cond replaces almost the entirety of:
    ::DBIHacks::_extract_fixed_condition_columns
    ::ResultSet::_collapse_cond
    ::ResultSet::_stack_cond
- Extra fixes for create/populate inheritance corner cases
- More predictable SQL condition generation order (which incidentally may
  prove problematic down the road with broken tests, but oh well, we'll
  burn when we get there)
- Ton of extra tests and corner cases

We even managed to fulfill a longstanding TODO, even though it is a *lucky*
side-effect and not a real fix. Making a note to address this later... sigh.

This work started from d8b7d9f58..337f3ee80, which while taking the right
direction had too many loose ends. ilmari++
The initial (now passing) test for RT#93244 (fixed in 8d005ad9)
Updated discard_changes docs regarding { force_pool => 'master' }
Improve complex order+prefetch exception message
This test was essentially c/p-ed in 2cc3a7be3, consolidate

The overriding difference is the new-connection == new-storage == new-sqla
Add an explicit codepath to test that, but overall there is plenty of
other places that verify this behavior to death

Examine under -w -C -M to make sense out of the consolidation (minimal
changes were attempted)
Reaching for IO::File here makes no sense
Consolidate lib-wide frameskip, adjust the ::RunMode loading order
Move the DSN-lock machinery from 8d6b1478d into DBICTest::BaseSchema

This is just a c/p job, expecting zero functional changes

The t/admin/02ddl.t are cosmetic changes fixing a nested &cref leak never
before encountered on <= 5.8.7
Less warning noise from sqlite test
Fix erroneous todoification in sqlite test
Fix multiple savepointing transactions on DBD::SQLite

Problem was missed during review of 86a51471ce (how the fuck did I let this
abomination through anyway), and not encountered due to insufficient testing.
The naive statement parser in DBD::SQLite when running against older libsqlite
mistakenly treats ROLLBACK TRANSACTION TO... as an actual TXN rollback, and
as a result desyncs the internal AutoCommit flag state [1]
Fix by simply using the shorter (still valid) syntax [2], and by removing the
sloppy workaround hiding the actual problem.

[1] https://github.com/DBD-SQLite/DBD-SQLite/blob/1.42/dbdimp.c#L824:L852
[2] http://www.sqlite.org/lang_savepoint.html
Consolidate pg/mysql and standalone SQLite savepoint tests to run on all 3

A larger consolidation will take place later with T::WV, but all in due time
Excise live test left over after ac0c082542

No access to a live test system currently, and likely the test is incorrect
anyway (no mention of significant parenthesis)