chore(ckbtc): measure event migration instruction count

[ninegua]

In #3434, empty events are being removed from events recorded in stable memory.

To be sure that this migration would succeed as part of the post_upgrade computation, its actual execution instruction counts needs to be measured.

[ninegua]

Dumping stable memory succeeded, but canbench doesn't seem to work as intended when indicating that stable memory should be loaded from a file.

Alternatively we can call finish_upload manually after uploading all events, and this function returns the instruction count (and number of empty events removed) as a return value.

$ cargo test --test dump_stable_memory -- test_minter_event_upload_mainnet --show-output --nocapture
...
finish upload = (27765179443, Some(409141))
test test_minter_event_upload_mainnet ... ok

So the instruction count is around 27.7 billion for calling storage::migrate_old_events_if_not_empty.
If we instead measure the whole post_upgrade (after modifying finish_upload function), the result is:

$ cargo test --test dump_stable_memory -- test_minter_event_upload_mainnet --show-output --nocapture
...
finish upload = (31556020665,)
test test_minter_event_upload_mainnet ... ok

This is well below the instruction limit for post_upgrade, which is 300 billion at the moment.