sql: make BtCursor's memory aligned

Vdbe at runtime allocates VdbeCursor structure using
allocateCursor() function. Inside there is a pointer at BtCursor
structure. To make the allocation faster and improve cache
locality, both cursors are allocated in one memory block + some
extra memory for uint32_t array, where BtCursor followed
VdbeCursor and the array without any padding:

   VdbeCursor + uint32_t * N + BtCursor

The problem is that BtCursor needs 8 byte alignment. When it
followed VdbeCursor (aligned by 8) + some uint32_t values, its
actual alignment could become 4 bytes. That led to a crash when
alignment sanitizer is enabled in clang.

The patch makes BtCursor offset aligned by 8 bytes.

Part of #4609