Disk reclaim on MongoDB WiredTiger Engine

This example shows that deleting MongoDB collection will release the disk space to OS ( only applicable for WT storage engine)

```
hasitha@DESKTOP / $ sudo du -sh /data/
6.0G    /data/

hasitha@DESKTOP / $ mongo_local
MongoDB shell version v3.4.21
connecting to: mongodb://localhost:27017/
MongoDB server version: 3.4.21
> show dbs
admin           0.000GB
central_db      0.000GB
e_1v6y03dpfs6z  0.001GB
e_2nas8av7dawh  5.552GB
e_da618q7mfzsv  0.001GB
e_p4rxh05m2k4w  0.008GB
e_testdb        0.001GB
e_yrrsst6ncktg  0.004GB
local           0.000GB
m_testdb        0.000GB
m_yrrsst6ncktg  0.000GB
s_testdb        0.000GB

> use e_2nas8av7dawh
switched to db e_2nas8av7dawh

> show collections
users

> db.users.drop()

true

> show collections

> exit

bye

hasitha@DESKTOP / $ sudo du -sh /data/

358M    /data/

```