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

v5.2

Fix typo causing docs build failure.

Fixes elastic#6488

Updated lock file for 5.1 (elastic#6356)

Updated lock file for 5.1 (elastic#6356)

use a temporary path instead of a temporary file

Using an empty file for the regitry make FB quits with a EOF error

Fixes: elastic#6285

Fixes elastic#6286

Regenerate lock file for 5.0.1 (elastic#6239)