multiline: refactor flush logic

[Syn3rman]

we are flushing the stream buffer in case it is
different in two consecutive runs, which leads to
incorrect parsing of multiline logs where we have
messages from different streams (stderr & stdout for ex)
interleaved for the cri parser. Note that for the docker
logs this is expected behaviour, but cri logs specify
F (full) and P (partial) logs, so we can parse multiline
logs even if lines from different streams are interleaved

Fixes #4387

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

[INPUT]
  Name tail
  Path ../cri.log
  Multiline.Parser cri
  Read_from_head True

[OUTPUT]
  name stdout
  match *

• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

Fluent Bit v2.1.3
* Copyright (C) 2015-2022 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2023/05/24 12:30:33] [ info] [fluent bit] version=2.1.3, commit=5559192d2c, pid=22318
[2023/05/24 12:30:33] [ info] [storage] ver=1.4.0, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2023/05/24 12:30:33] [ info] [cmetrics] version=0.6.1
[2023/05/24 12:30:33] [ info] [output:stdout:stdout.0] worker #0 started
[2023/05/24 12:30:33] [ info] [ctraces ] version=0.3.1
[2023/05/24 12:30:33] [ info] [input:tail:tail.0] initializing
[2023/05/24 12:30:33] [ info] [input:tail:tail.0] storage_strategy='memory' (memory only)
[2023/05/24 12:30:33] [ info] [input:tail:tail.0] multiline core started
[2023/05/24 12:30:33] [ info] [sp] stream processor started
[2023/05/24 12:30:33] [ info] [input:tail:tail.0] inotify_fs_add(): inode=271312 watch_fd=1 name=../cri.log
[0] tail.0: [[1637246854.730908438, {}], {"time"=>"2021-11-18T14:47:34.730908438+00:00", "stream"=>"stdout", "_p"=>"P", "log"=>"start Bend B"}]
[1] tail.0: [[1637246854.735454549, {}], {"time"=>"2021-11-18T14:47:34.735454549+00:00", "stream"=>"stderr", "_p"=>"F", "log"=>"some log message D in stderr part one"}]
[2] tail.0: [[1637246854.730939216, {}], {"time"=>"2021-11-18T14:47:34.730939216+00:00", "stream"=>"stdout", "_p"=>"P", "log"=>"C 1message C still continued after a line from a different stream"}]
^C[2023/05/24 12:30:35] [engine] caught signal (SIGINT)
[2023/05/24 12:30:35] [ warn] [engine] service will shutdown in max 5 seconds
[2023/05/24 12:30:35] [ info] [input] pausing tail.0
[2023/05/24 12:30:35] [ info] [engine] service has stopped (0 pending tasks)
[2023/05/24 12:30:35] [ info] [input] pausing tail.0
[2023/05/24 12:30:35] [ info] [output:stdout:stdout.0] thread worker #0 stopping...
[2023/05/24 12:30:35] [ info] [input:tail:tail.0] inotify_fs_remove(): inode=271312 watch_fd=1
[2023/05/24 12:30:35] [ info] [output:stdout:stdout.0] thread worker #0 stopped
==22318==
==22318== HEAP SUMMARY:
==22318==     in use at exit: 0 bytes in 0 blocks
==22318==   total heap usage: 1,771 allocs, 1,771 frees, 1,136,978 bytes allocated
==22318==
==22318== All heap blocks were freed -- no leaks are possible
==22318==
==22318== For lists of detected and suppressed errors, rerun with: -s
==22318== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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