RFR: 8269064: Dropped messages of AsyncLogWriter cause memleak

[Xin Liu]

free c-strings of the dropped messages.

This patch added KVHashtable::remove(K), which allows user to remove the 
useless entries from the hashtable. Remove the corresponding entry from
AsyncLogWriter::_stats when the output is about to delete.

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Commit messages:
 - Remove the entry of output in AsyncLogWriter when delete the output.
 - 8269064: Dropped messages of AsyncLogWriter cause memleak

Changes: https://git.openjdk.java.net/jdk/pull/4537/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4537&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269064
  Stats: 143 lines in 6 files changed: 142 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4537.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4537/head:pull/4537

PR: https://git.openjdk.java.net/jdk/pull/4537