Use Lua scripts for redis key counting

[MartinKolarik]

Right now, in updateMeasurementCount(), we iterate over all measurements using a default scan size, which is ten records. That means about 7k calls just to count the keys. We could use a larger batch size as in store cleanup() but since we don't need the keys at all, just their count, it's still a lot of unnecessary IO. We should replace this with a small lua script, similar to this that just returns the number of matching keys. The redis client has built-in support for script handling: https://github.com/redis/node-redis#lua-scripts

[alexey-yarmosh]

As I see we may also benefit from using lua scripts in places with a chain of calls where next call is based on the data of the previous one:

• https://github.com/jsdelivr/globalping/blob/master/src/measurement/runner.ts#L53-L62

And in places with multiple parallel redis operations:

• https://github.com/jsdelivr/globalping/blob/master/src/measurement/store.ts#L36-L49