RFR: 8293562: blocked threads with KeepAliveCache.get

[Daniel Jeliński]

Please review this patch that makes sure KeepAliveCache does not block all threads while closing sockets.

Changes:
- get operation no longer closes sockets; if there's no socket that is recent enough, get returns null and lets the cleaner thread close the sockets
- put operation closes sockets without holding the cache lock. Additionally, if the cache is full, it places the new connection in the cache and removes the oldest connection.
- the cleaner thread creates a list of connections to close, and then closes them after releasing the cache lock
- additionally, we set the socket timeout to 1 millisecond before calling socket.close

The new test fails with `Wait for second request timed out` without this patch, passes after the changes. Tiers 1-3 clean.

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Commit messages:
 - Use shorter timeout when closing connection
 - Release locks before closing connections
 - Close connections outside KAC lock
 - Remove unused method

Changes: https://git.openjdk.org/jdk/pull/10401/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10401&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293562
  Stats: 338 lines in 3 files changed: 272 ins; 40 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/10401.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10401/head:pull/10401

PR: https://git.openjdk.org/jdk/pull/10401