feat: optimize tcmalloc release memory

[hycdong]

What this pr solved

Pull request #278 add a new feature that releasing memory periodically by tcmalloc ReleaseFreeMemory. ReleaseFreeMemory will release all tcmalloc reserved and not-used memory back to operating system. Releasing memory will lock page heap, the structure used to allocate memory from operating system and release memory back to operating system, shared by all threads. As a result, releasing all free memory will lock page heap for a long time and block all threads, more seriously, it will affect cluster performance. More tcmalloc information can be find in tcmalloc website.

So we optimize the releasing memory method, which is similar to kudu's way (kudu gc_memory).
We use tcmalloc statistics below:

'tcmalloc.pageheap_free_bytes' - tcmalloc reserved but not-used memory
'generic.current_allocated_bytes' - application whole allocated memory

We set a maximum percentage p, we assume that tcmalloc reserved but not-used memory should not be greater whole_used_memory * p, if exceed this threshold, we will release the exceeding memory by ReleaseToSystem. To avoid locking page heap for a long time, we only release 1MB at a time.

Besides, we make such percentage mem_release_max_reserved_mem_percentage configurable dynamically. Cluster administrator can show and change this value by remote_command.

>>> remote_command -t replica-server replica.mem-release-max-reserved-percentage 5
COMMAND: replica.mem-release-max-reserved-percentage 5

CALL [replica-server] [ip1:port1] succeed: OK
CALL [replica-server] [ip2:port2] succeed: OK
CALL [replica-server] [ip3:port3] succeed: OK

Succeed count: 3
Failed count: 0

>>> remote_command -t replica-server replica.mem-release-max-reserved-percentage
COMMAND: replica.mem-release-max-reserved-percentage

CALL [replica-server] [[ip1:port1] succeed: mem-release-max-reserved-percentage=5
CALL [replica-server] [[ip2:port2] succeed: mem-release-max-reserved-percentage=5
CALL [replica-server] [[ip3:port3] succeed: mem-release-max-reserved-percentage=5

Succeed count: 3
Failed count: 0

Config updates

Add two new config

mem_release_check_interval_ms = 3600000;
mem_release_max_reserved_mem_percentage = 10;

remove config mem_release_interval_ms

[neverchanje]

https://github.com/apache/kudu/blob/master/src/kudu/util/process_memory-test.cc kudu 的单测为什么不一并搬过来？

[hycdong]

https://github.com/apache/kudu/blob/master/src/kudu/util/process_memory-test.cc kudu 的单测为什么不一并搬过来？

这个单测测试的kudu的函数CurrentConsumption，没有GcTcmalloc的单测

[neverchanje]

这个 remote command 还是建议抽出来一个函数然后写单测

[acelyc111]

+1 这几处代码都很类似，可以抽象出一个函数来， 几个remote command都复用。另起一个PR做吧

[hycdong]

嗯，remote command整个可以抽出来，之后的pr再做吧