Cleanup SlicePositionsAppender

[lukasz-stec]

Description

Is this change a fix, improvement, new feature, refactoring, or other?

improvement

Is this a change to the core query engine, a connector, client library, or the SPI interfaces? (be specific)

core query engine

How would you describe this change to a non-technical end user or system administrator?

Code refactoring

Related issues, pull requests, and links

In response to discussion here

Documentation

( ) No documentation is needed.
( ) Sufficient documentation is included in this PR.
( ) Documentation PR is available with #prnumber.
( ) Documentation issue #issuenumber is filed, and can be handled later.

Release notes

( ) No release notes entries required.
( ) Release notes entries required with the following suggested text:

# Section
* Fix some things. ({issue}`issuenumber`)

[lukasz-stec]

I ran a subset of BenchmarkPartitionedOutputOperator jmh to confirm no regressions cc @sopel39

before
Benchmark                                   (channelCount)  (enableCompression)  (nullRate)  (partitionCount)  (positionCount)   (type)  Mode  Cnt     Score    Error  Units
BenchmarkPartitionedOutputOperator.addPage               1                false           0                16             8192  VARCHAR  avgt   10  2218.089 ± 32.786  ms/op
BenchmarkPartitionedOutputOperator.addPage               1                false         0.2                16             8192  VARCHAR  avgt   10  2028.034 ± 24.532  ms/op

after
Benchmark                                   (channelCount)  (enableCompression)  (nullRate)  (partitionCount)  (positionCount)   (type)  Mode  Cnt     Score    Error  Units
BenchmarkPartitionedOutputOperator.addPage               1                false           0                16             8192  VARCHAR  avgt   10  2217.335 ± 30.768  ms/op
BenchmarkPartitionedOutputOperator.addPage               1                false         0.2                16             8192  VARCHAR  avgt   10  2035.245 ± 29.503  ms/op

[lukasz-stec]

@sopel39 I divided the changes in separate commits