Feature: implement the counter

[empiredan]

In #922 it has been mentioned that a counter in essence is a 64-bit integer that can be incremented and decremented. It can be used to measure the number of tasks in queues, current number of running manual compacts, etc. All counters start out at 0.

From the aspect of implementations, there are 2 kinds of counters:

• One is the general type of Counter that are implemented by striped_long_adder, which can achieve high performance while consuming less memory if it's not updated very frequently.
• Another uses concurrent_long_adder as the underlying implementation. It has higher performance while consuming more memory if it's updated very frequently.

For the details of striped_long_adder and concurrent_long_adder please see #889.