wallet: bracket WriteHDMaster in a SQLite runtime transaction

The SQLite runtime wrote each record as its own autocommit, so a group
that must land together -- hdmaster and hdchaincode, the two halves of an
HD seed -- could be left half-written by a failure or crash between them,
the incomplete-seed state -walletstoragecheck exists to catch. Berkeley DB
was no worse (it is not transactional here either) but no better.

Add WalletSQLiteRuntimeBeginTxn/CommitTxn/RollbackTxn -- no-ops that
succeed when the SQLite backend is not active, so Berkeley DB keeps its
existing per-write behaviour -- and wrap WriteHDMaster in them so the seed
lands all-or-nothing under SQLite. The one BDB-atomic wallet rewrite, the
encrypt path, is already matched by its own transaction in the SQLite
encrypt branch; every other wallet multi-write is ordering-based on both
backends by design.

wallet-sqlite self-test now proves the bracket: a rolled-back runtime
write leaves no record, a committed one persists, and WriteHDMaster lands
both halves of the seed.

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