update RecordIOWriter javadoc to further describe file-structure

bump version to 5.3.3 (elastic#7171)

backport agent_spec fixes to 5.x

Fixes elastic#7146

Add changes from the review

Fixes elastic#6988

Merge pull request elastic#6993 from dedemorton/cherrypick_#6832_5.4

Cherrypick elastic#6832 into 5.4

bump tp 5.3.2 (elastic#6942)

fix PQ write concurrency issues and ligering files

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move nextSeqNum call under mutex lock to prevent seqNum race condition

fully acked head page beading should not create new tail page

explicit purge required to clean physical files

correctly remove preserved checkpoints

small review changes

single element write can only really wakeup a single thread

Fixes elastic#6800

Add note about JAVA_HOME to docs

Based on a suggestion to update the documentation: https://discuss.elastic.co/t/logstash-not-seeing-conf-files-in-conf-d-dir/75931/9

Fixes elastic#6751

Support params to specify output dir

Fixes elastic#6682