model: flatten "pending acquire/release sequence" structure

model: flatten "pending acquire/release sequence" structure

This structure should just be a simple flat array, so that we can simply
iterate through all acquire operations with pending release sequences. There
probably wasn't much to be saved by organizing this structure as a hash table
of lists; if so, I can add the complexity back in later.

This eliminates the need for a separate 'lazy_sync_size' count and renames
'lazy_sync_with_release' to 'pending_acq_rel_seq'.