Btree interface refactoring:

Btree interface refactoring:
(1) The sqlite3BtreeKeySize() interface is renamed to sqlite3BtreeIntegerKey()
and modified to work only for table btrees with a rowid.
(2) The sqlite3BtreeDataSize() interface is renamed to sqlite3BtreePayloadSize()
and modified to work with any btree.
(3) The sqlite3BtreeDataFetch() and sqlite3BtreeKeyFetch() routines are
combined into a single sqlite3BtreePayloadFetch() routine.
The result of these changes is a smaller binary and fewer CPU cycles needed
to run queries.

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