Feature enhancement: support release all tcmalloc reserved not-used memory

[hycdong]

Feature Request

Pegasus now support trying to release tcmalloc reserved not-used memory periodically. We define a max-reserved percentage, if the reserved not-used memory exceed the threshold, server will release the exceeded memory back to operating system. However, sometimes memory increase repaidly before the check triggered. So we need to add a command to release all reserved not-used memory immediately when system memory is seriously short.
NOTICE: Strongly recommend using this command when seriously lack of memory, releasing all reserved memory may affect performance.