Use a ThreadPool for remote /metrics/find requests

2 new config parameters:
  - REMOTE_STORE_USE_THREADS: Boolean, default to false
  - REMOTE_STORE_THREADPOOL_SIZE: number of threads to use, defaults to 8
Close connection after fetch

Without this, the connection remained open in the fetcher threads.
Add threads support to find_first
Use a ThreadPool in fetchData()

controlled by REMOTE_STORE_USE_THREADS and REMOTE_STORE_THREADPOOL_SIZE
It will use more memory when threads are disabled since all the dbFile.fetch()
results will be stored in memory at the same time.