sdk: implement AccountSharedData::is_default()

[2501babe]

Problem

in #3146, we need to check if an AccountSharedData is equal to the default, but account == AccountSharedData::default() performs a useless heap allocation for the Arc<Vec<u8>> control data

Summary of Changes

implement AccountSharedData::is_default(), which avoids this allocation

(will hold off merging until 2.1 is forked)

[2501babe]

closing as no longer needed, we can just compare lamports after all. i was being overly cautious, starry points out that there should never be a zero-lamport read-only account to shadow, because the only way an account could get to zero lamports is by being writable, triggering garbage collection