Summary:
After secondary instance replays the logs from primary, certain files become
obsolete. The secondary should find these files, evict their table readers from
table cache and close them. If this is not done, the secondary will hold on to
these files and prevent their space from being freed.
Test plan (devserver):
```
$./db_secondary_test --gtest_filter=DBSecondaryTest.SecondaryCloseFiles
$make check
$./db_stress -ops_per_thread=100000 -enable_secondary=true -threads=32 -secondary_catch_up_one_in=10000 -clear_column_family_one_in=1000 -reopen=100
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6114
Differential Revision: D18769998
Pulled By: riversand963
fbshipit-source-id: 5d1f151567247196164e1b79d8402fa2045b9120
fe1147db1c