Cosmetic rearranging/renaming of some parts of the populate codepath

Zero functional changes
Fix both a dubious test and a regression in populate args immutability

The rewrite in d0cefd99 optimized populate enough that the same data structure
would now make it all the way to the stringifier in ::Storage. This was not
caught due to a deficient test.

Read diff under -w for maximum sense
Deprecate insert_bulk - we will be changing its signature down the road

Besides there is really no reason for users to call it directly
Fix $rs->populate with column name array but no row data

Two regressions for both context types snuck in during the the big populate()
rewrite d0cefd99

- In scalar context the return value incorrectly went from () to []
- In void context an empty $data was handed to ::Storage::_insert_bulk
Simplify some ::Storage::DBI methods (no tempvars)

No functional changes at all
Tighten the 'txn-op on disconnected handle' checks
S::L tests rely on preserving the sqlmaker between reconnections

While this is utterly retarded, punt and backout the b6a469f7 change for now.
Will revisit again during the connect_info consolidation