Keep track of data reallocations in BorrowedAccount

[seanyoung]

Problem

With direct mapping, we clear the spare capacity in the data Vec when the account size shrinks. When a reallocation happens in Vec, the data maybe copied to a new location, but the spare capacity in the Vec will no longer be cleared. So, keep track of reallocations.

Summary of Changes

[mergify]

The Firedancer team maintains a line-for-line reimplementation of the
native programs, and until native programs are moved to BPF, those
implementations must exactly match their Agave counterparts.
If this PR represents a change to a native program implementation (not
tests), please include a reviewer from the Firedancer team. And please
keep refactors to a minimum.