AVRO-4067: Optimize First Byte of Long Decode

[belugabehr]

What is the purpose of the change

• This pull request improves file read performance by optimizing the first byte of varint Longs, fixing AVRO-407

Verifying this change

This change is a trivial rework / code cleanup without any test coverage. See ticket for JMH results

[belugabehr]

For consideration.

[belugabehr]

@martin-g How about this one? :)

[belugabehr]

@martin-g Can you please take a look at this one when you get some time?

[belugabehr]

@martin-g As always, in your debt. Please take a look when you can.

[martin-g]

in your debt

Not really to me! But yes, if you want to pay some debt you can start reviewing & merging others' PRs too! ;-) I'll take a look at your PRs in the next days.