fix: R5a. and reuse released addr

[blake-regalia]

Removes the use of random addresses when mutating the buffer. Instead, reuses existing addresses in order to reduce the signal to noise ratio that random addresses would otherwise produce in storage access patterns. Also applies this logic when releasing the owner/sender.

Guiding pseudocode for the if/else logic in choosing presumptive_settle_index and write_index:

if recipient_in_buffer:
	settle the dummy entry
	update the existing recipient's buffered balance
else:
	if undersaturated:
		settle dummy entry
		place new entry at next pos
	else:
		settle random entry
		replace settled entry with new entry