Restore SQLite::Savepoint ABI compatibility

[SRombauts]

Summary

• remove the mbRolledBack data member added by Fix Savepoint destructor exception-safety and rollback bookkeeping #559
• restore the SQLiteCpp 3.x SQLite::Savepoint object layout
• keep the broader catch (...) destructor exception-safety fix from Fix Savepoint destructor exception-safety and rollback bookkeeping #559
• retain the previous harmless repeated ROLLBACK TO behavior when a savepoint was manually rolled back but not released

Rationale

SQLite::Savepoint is a public class instantiated by value by consumers. Adding a private data member changes its class layout and can break strict binary compatibility while SQLiteCpp 3.4.0 still keeps the 3.x ABI / SOVERSION 0.

The additional rollback state was only an optimization to avoid issuing a second ROLLBACK TO from the destructor. SQLite already tolerated the previous behavior, so restoring that implementation is the smallest compatibility-preserving fix before 3.4.0.

Validation

• both commits remain buildable independently
• existing Savepoint tests cover manual rollback/release and destructor behavior
• CI will exercise the full supported build matrix

[coveralls]

coverage: 99.713%. remained the same — fix-savepoint-abi into master