RFR: 8286739: Refactor FreeListConfig

[Albert Mingkun Yang]

This PR consists of two commits:

1 is about making `FreeListConfig` an "interface" of `allocate/deallocate`.
2 is about renaming `FreeListConfig -> MemAllocator`.

Test: hotspot_gc

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

Commit messages:
 - rename
 - split config base class

Changes: https://git.openjdk.java.net/jdk/pull/8704/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8704&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286739
  Stats: 69 lines in 5 files changed: 18 ins; 26 del; 25 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8704.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8704/head:pull/8704

PR: https://git.openjdk.java.net/jdk/pull/8704