[OTFC] Move mmap initializing into Config

Summary: The mmap initializing should belong to config structure.

Test Plan:
```$ bin/gocov test github.com/gow/otfc/ | tee /tmp/qqq/1 | bin/gocov report | sort
ok    github.com/gow/otfc 0.100s

github.com/gow/otfc/     ---------------------------   66.18% (90/136)
github.com/gow/otfc//config.go   Config.delete       100.00% (1/1)
github.com/gow/otfc//config.go   Config.get      100.00% (4/4)
github.com/gow/otfc//config.go   Config.set      87.50% (14/16)
github.com/gow/otfc//config.go   InitConfig      60.00% (9/15)
github.com/gow/otfc//data.go   dataBlock.get       60.00% (3/5)
github.com/gow/otfc//data.go   dataBlock.set       66.67% (4/6)
github.com/gow/otfc//errors.go   ConfigError.ErrNo     100.00% (1/1)
github.com/gow/otfc//errors.go   ConfigError.Error     0.00% (0/7)
github.com/gow/otfc//header.go   configHeader.ByteSize     0.00% (0/1)
github.com/gow/otfc//header.go   configHeader.ModifiedTime   0.00% (0/1)
github.com/gow/otfc//header.go   configHeader.NumRecords   100.00% (1/1)
github.com/gow/otfc//header.go   configHeader.print    0.00% (0/6)
github.com/gow/otfc//header.go   configHeader.SetRecordCount   100.00% (2/2)
github.com/gow/otfc//header.go   configHeader.SetVersion   100.00% (2/2)
github.com/gow/otfc//header.go   configHeader.UpdateTime   100.00% (1/1)
github.com/gow/otfc//header.go   configHeader.Version    100.00% (1/1)
github.com/gow/otfc//index.go  getKeyHash      100.00% (5/5)
github.com/gow/otfc//index.go  indexBlock.delete     77.78% (7/9)
github.com/gow/otfc//index.go  indexBlock.find     100.00% (6/6)
github.com/gow/otfc//index.go  indexBlock.get      71.43% (5/7)
github.com/gow/otfc//index.go  indexBlock.print    0.00% (0/1)
github.com/gow/otfc//index.go  indexBlock.set      100.00% (10/10)
github.com/gow/otfc//index.go  indexRecord.print     0.00% (0/1)
github.com/gow/otfc//otfc.go   createFile      58.33% (7/12)
github.com/gow/otfc//otfc.go   Delete        100.00% (1/1)
github.com/gow/otfc//otfc.go   Get         100.00% (1/1)
github.com/gow/otfc//otfc.go   Init        100.00% (2/2)
github.com/gow/otfc//otfc.go   Print         0.00% (0/4)
github.com/gow/otfc//otfc.go   PrintHeaderBlock    0.00% (0/1)
github.com/gow/otfc//otfc.go   PrintIndexBlock     0.00% (0/1)
github.com/gow/otfc//otfc.go   Set         100.00% (1/1)
github.com/gow/otfc//otfc.go   Shutdown      50.00% (2/4)```

Reviewers: tommy.testy

Reviewed By: tommy.testy

Differential Revision: http://dev.chetan.io/D32