output: memoize calculate_timekey to reduce redundant calculations

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
calculate_timekey is called for every emitted record.
Since logs usually arrive chronologically, consecutive records almost always fall into the same timekey.
However, the current implementation performs % and to_i for every single call.

When running rake benchmark:run:in_tail with 10 GB of data, this memoization improved the overall throughput:

Benchmark	before	after
rake benchmark:run:in_tail	43.170137	41.584905

Docs Changes:
N/A

Release Note:

• output: Improve performance by memoizing calculate_timekey

[kenhys]

Thanks