Cache compiled regular expressions in the log buffer

[DigitalVeer]

Summary

The log buffer keeps each compiled regular expression in a cache.
The cache size is not more than the number of patterns given to Match.
This PR removes the TODO in log_buffer.go.

Benchmark

One Match call with a default kernel monitor pattern:

	Time per call	Memory per call	Allocations per call
Before	2590 ns	9265 B	49
After	211 ns	776 B	1

NONE

[kubernetes-prow]

Hi @DigitalVeer. Thanks for your PR.

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[wangzhen127]

/ok-to-test

[hakman]

Thanks @DigitalVeer, LGTM!

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: DigitalVeer, hakman

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[hakman]

/test pull-npd-build