Page storage bug fix (pingcap#87)

* add test cases for PageStorage

* split PageStorage gc stage into small helper functions

* add test cases for PageStorage gc concurrency

* add stress test and dump utils of PageStorage

* Fix bug:
1. ensure PageFile with meta && data, in case gc drop file but be killed before drop dir
2. turn PageStorage::Config::sync_on_write = true by default
3. avoid PageStorage::gc run by multi-threads
4. print PageFile's path if checksum is not correct

* throw exception for must_exist==false and errno != ENOENT

dbms/src/Storages/Page/tests/utils_get_valid_pages.cpp

@@ -100,4 +100,3 @@ int main(int argc, char ** argv)
 
     return 0;
 }
-