* Fix bug in usage metrics that caused a negative count to occur
There were a couple of instances were usage metrics would do the wrong
thing and result in incorrect counts, causing the count to attempt to
decrement below zero and return an error. The usage metrics did not
account for various places where a single transaction could
delete/update/add multiple service instances at once.
We also remove the error when attempting to decrement below zero, and
instead just make sure we do not accidentally underflow the unsigned
integer. This is a more graceful failure than returning an error and not
allowing a transaction to commit.
* Add changelog
This commit refactors the state store usage code to track unique service
name changes on transaction commit. This means we only need to lookup
usage entries when reading the information, as opposed to iterating over
a large number of service indices.
- Take into account a service instance's name being changed
- Do not iterate through entire list of service instances, we only care
about whether there is 0, 1, or more than 1.
We add a WriteTxn interface for use in updating the usage memdb table,
with the forward-looking prospect of incrementally converting other
functions to accept interfaces.
As well, we use the ReadTxn in new usage code, and as a side effect
convert a couple of existing functions to use that interface as well.
