Lock contentions by small filemgr-related functions

After we partition the block cache into multiple shards, a spinlock 'file->lock' becomes a major bottleneck under multi-threaded workloads.

ForestDB's readers frequently call below internal small functions which grab file->lock and incur lock contentions:

filemgr_is_writable()
filemgr_get_pos()
filemgr_get_file_status()
filemgr_dirty_root_exist()
filemgr_get_header_bid()

We need to use atomic variables to eliminate lock contentions.