IDB: Move Database::PutOperation into Transaction.

Put is part of the Transaction mojo and uses a bunch of
BackingStore::Transaction interface methods. It's not causing a lot of
harm in Database, but moving it makes the code a bit more encapsulated
and legible.

Bug: none

Change-Id: If6dbeeed80acd46114d58d3e66199a210c9f8e11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6507110
Reviewed-by: Abhishek Shanthkumar <abhishek.shanthkumar@microsoft.com>
Commit-Queue: Evan Stade <evanstade@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1457051}