[DyConf] fixed the delete bug

Summary: Deleting a key never decremented the index counter. Fixed it.

Test Plan:
```
$ bin/gocov test -deps -exclude-goroot github.com/gow/dyconf/ | tee /tmp/qqq | bin/gocov report
ok    github.com/gow/dyconf 0.139s

github.com/gow/dyconf/otfc.go  Init    100.00% (2/2)
github.com/gow/dyconf/otfc.go  Set     100.00% (1/1)
github.com/gow/dyconf/otfc.go  Get     100.00% (1/1)
github.com/gow/dyconf/otfc.go  Delete    100.00% (1/1)
github.com/gow/dyconf/otfc.go  Shutdown  50.00% (2/4)
github.com/gow/dyconf/otfc.go  Print     0.00% (0/4)
github.com/gow/dyconf    --------  53.85% (7/13)

github.com/gow/dyconf/config/index.go  indexBlock.set      100.00% (10/10)
github.com/gow/dyconf/config/index.go  indexBlock.find     100.00% (6/6)
github.com/gow/dyconf/config/index.go  indexBlock.delete     100.00% (6/6)
github.com/gow/dyconf/config/index.go  getKeyHash      100.00% (5/5)
github.com/gow/dyconf/config/config.go   ConfigFile.Get      100.00% (4/4)
github.com/gow/dyconf/config/header.go   configHeader.SetRecordCount   100.00% (2/2)
github.com/gow/dyconf/config/header.go   configHeader.SetVersion   100.00% (2/2)
github.com/gow/dyconf/config/header.go   configHeader.UpdateTime   100.00% (1/1)
github.com/gow/dyconf/config/config.go   NewWritable       100.00% (1/1)
github.com/gow/dyconf/config/header.go   configHeader.NumRecords   100.00% (1/1)
github.com/gow/dyconf/config/config.go   ConfigFile.Delete     100.00% (1/1)
github.com/gow/dyconf/config/header.go   configHeader.Version    100.00% (1/1)
github.com/gow/dyconf/config/config.go   ConfigFile.Set      87.50% (14/16)
github.com/gow/dyconf/config/index.go  indexBlock.get      71.43% (5/7)
github.com/gow/dyconf/config/data.go   dataBlock.set       66.67% (4/6)
github.com/gow/dyconf/config/config.go   initWithProtOptions     60.00% (9/15)
github.com/gow/dyconf/config/data.go   dataBlock.get       60.00% (3/5)
github.com/gow/dyconf/config/utils.go  createFile      58.33% (7/12)
github.com/gow/dyconf/config/errors.go   Error.ErrorString     0.00% (0/7)
github.com/gow/dyconf/config/header.go   configHeader.print    0.00% (0/6)
github.com/gow/dyconf/config/config.go   New         0.00% (0/1)
github.com/gow/dyconf/config/errors.go   Error.Error       0.00% (0/1)
github.com/gow/dyconf/config/errors.go   Error.GetErrorNo    0.00% (0/1)
github.com/gow/dyconf/config/errors.go   Error.GetErrorString    0.00% (0/1)
github.com/gow/dyconf/config/header.go   configHeader.ByteSize     0.00% (0/1)
github.com/gow/dyconf/config/header.go   configHeader.ModifiedTime   0.00% (0/1)
github.com/gow/dyconf/config/index.go  indexRecord.print     0.00% (0/1)
github.com/gow/dyconf/config/index.go  indexBlock.print    0.00% (0/1)
github.com/gow/dyconf/config/utils.go  ConfigFile.PrintIndexBlock  0.00% (0/1)
github.com/gow/dyconf/config/utils.go  ConfigFile.PrintHeaderBlock   0.00% (0/1)
github.com/gow/dyconf/config     ---------------------------   66.13% (82/124)
```

Reviewers: tommy.testy

Reviewed By: tommy.testy

Maniphest Tasks: T10

Differential Revision: D45