Revert the recent fixes to backup code and instead fix the problem reported by forum...

Revert the recent fixes to backup code and instead fix the problem reported by forum post [forum:15d82885e2 | 15d82885e2] (that a call to sqlite3_deserialize() after sqlite3_backup_init() but before the first call to sqlite3_backup_step() on the destination db of a backup could cause a crash) by deferring caching the pointer to the destination Btree until after it is locked.

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