RFR: 8294037: Using alias template to unify hashtables in AsyncLogWriter

[Xin Liu]

This change is only for clear code. No functional change is intended.

Currently, there are two hashtables. One is this on c-heap.

  using AsyncLogMap = ResourceHashtable<LogFileStreamOutput*,
                          uint32_t,
                          17, /*table_size*/
                          ResourceObj::C_HEAP,
                          mtLogging>;

and the other one is its mirror on ResourceArena. It's not accident. The 2nd one is the snapshot of prior one. 
We use this approach to release lock early. Flushing snapshot doesn't require lock protection and therefore 
won't block logsites.


  ResourceMark rm;
  // Similar to AsyncLogMap but on resource_area
  ResourceHashtable<LogFileStreamOutput*, uint32_t,
                          17/*table_size*/, ResourceObj::RESOURCE_AREA,
                          mtLogging> snapshot;

C++11 has a new feature called [alias template](https://en.cppreference.com/w/cpp/language/type_alias). We can
use this feature to unify two types.

-------------

Commit messages:
 - 8294037: Using alias template to unify hashtables in AsyncLogWriter

Changes: https://git.openjdk.org/jdk/pull/10350/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10350&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294037
  Stats: 12 lines in 2 files changed: 3 ins; 5 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/10350.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10350/head:pull/10350

PR: https://git.openjdk.org/jdk/pull/10350