Cherry-pick #22974 to 7.x: Improve event normalization performance

[urso]

Cherry-pick of PR #22974 to 7.x branch. Original message:

Enhancement

What does this PR do?

Improve event normalization performance

Why is it important?

generalize/normalize is widely used in beats

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

How to test this PR locally

Benchmark

Related issues

Use cases

Screenshots

Logs

$ benchstat old.txt new.txt                                                                              
name             old time/op    new time/op    delta
Normalization-8     984ns ±16%     559ns ±13%  -43.16%  (p=0.000 n=20+19)

name             old alloc/op   new alloc/op   delta
Normalization-8    1.18kB ± 0%    0.74kB ± 0%  -37.41%  (p=0.000 n=20+20)

name             old allocs/op  new allocs/op  delta
Normalization-8      13.0 ± 0%       5.0 ± 0%  -61.54%  (p=0.000 n=20+20)

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Pinging @elastic/integrations-services (Team:Services)

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Build stats

• Build Cause: Pull request #23029 updated

• Start Time: 2020-12-14T13:34:46.429+0000

• Duration: 54 min 1 sec

Test stats 🧪

Test	Results
Failed	0
Passed	17385
Skipped	1404
Total	18789

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	17385
Skipped	1404
Total	18789