Proposed changes

BE can not graceful exit because some threads are running in endless
loop. This patch do the following optimization:

• Use the well encapsulated Thread and ThreadPool instead of std::thread
  and std::vectorstd::thread
• Use CountDownLatch in thread's loop condition to avoid endless loop
• Introduce a new class Daemon for daemon works, like tcmalloc_gc,
  memory_maintenance and calculate_metrics
• Decouple statistics type TaskWorkerPool and StorageEngine notification
  by submit tasks to TaskWorkerPool's queue
• Reorder objects' stop and deconstruct in main(), i.e. stop network
  services at first, then internal services
• Use libevent in pthreads mode, by calling evthread_use_pthreads(),
  then EvHttpServer can exit gracefully in multi-threads
• Call brpc::Server's Stop() and ClearServices() explicitly

Types of changes

What types of changes does your code introduce to Doris?
Put an x in the boxes that apply

• [] Bugfix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [] Documentation Update (if none of the other choices apply)
• Code refactor (Modify the code structure, format the code, etc...)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [] I have create an issue on (Fix #ISSUE), and have described the bug/feature there in detail
• Compiling and unit tests pass locally with my changes
• [] I have added tests that prove my fix is effective or that my feature works
• [] If this change need a document change, I have updated the document
• [] Any dependent changes have been merged

Further comments

none