TS-2168: Use #if condition to control reclaimable freelist options

1) Define reclaimabe freelist options only when compiles ATS
   with --enable-reclaimabe-freelist,

2) "proxy.config.allocator.enable_reclaim" will be set to 1 by default
   for two reasons:
   a) It should keep consistent with the description in
      records.config.default.in,
   b) More important, most users want to get reclaim feature when
      they compiles ATS with --enable-reclaimable-freelist.

3) Make the description in records.config.default.in more clearly.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>