Flush store data to temp file in BGREWRITEAOF command and use fsync

[ashwaniYDV]

Improve BGREWRITEAOF command (Issue is picked from TODO part in codebase)

• Make it safe from failure, an stable policy would be to write the new flushes to a temporary files and then rename them to the main process's AOF file
• Add fsync() and fdatasync() to persist to AOF for above cases.

Improvements

• If BGREWRITEAOF fails, no data gets lost as the old AOF will be untouched.