Refactor freelist management & add more dedicated tests

## Backgroud
There are some long standing data corruption issues, which indicate that there might be potential bug(s) in freelist management.
- https://github.com/etcd-io/bbolt/issues/778
- https://github.com/etcd-io/bbolt/issues/402
- https://github.com/etcd-io/bbolt/issues/705
- ...

I am not satisfied with the bbolt freelist management for a long time. It's hard to understand and also tightly coupled with bbolt. I have been thinking to refactor it to improve the understandability & testability.

## Refactor
The high level idea is to
- simplify the implementation to improve **understandability**;
- and introduce interface and decouple it with the bbolt TXN workflow to improve **testability**.

What we have done and are going to do:
- [x] https://github.com/etcd-io/bbolt/pull/777
- [x] https://github.com/etcd-io/bbolt/pull/783
- [x] https://github.com/etcd-io/bbolt/pull/775
- [x] https://github.com/etcd-io/bbolt/pull/788
- [x] https://github.com/etcd-io/bbolt/pull/792
- [ ] Simplify & update the logic related to the pending released free pages, refer to [doc](https://docs.google.com/spreadsheets/d/1T7ORCbv3mguq1hQoZkUg0XEzuUXFrNFFj1sUfQysZ2k/edit?gid=0#gid=0)

We also need to continue to refactor & simplify the interface from user (bbolt) perspective, in other words, we should have a clear understanding on how the interface will & should be used by bbolt. The motivation is to improve testablity.

The freelist management is the most sensitive & important part. So let's do it step by step.

## Test
Any unit tests are welcome.

But more importantly, we should add dedicated **randomized** test case to simulate **concurrent** (multiple) read TXNs and (single) writing TXN.
- We need to record all the requests or operations sent to the freelist module;
- We need to set up a list of expected behavior or invariable properties, and verify all the invariable properties are not broken during the test.