MB-9730 Add support for couch_file to refresh EOF

The couch_file process and its child writer process keeps track of
EOF information. This EOF position is incremented when data is
appended to the file by the writer process. The reader also
makes use of stored EOF position for reading latest header from
the current file process.

When an external process opens the same file and appends data into
the file, the EOF information held by the couch_file process becomes
invalid. Hence, we need to update the EOF information held by the
couch_file process explicitly.

Added couch_file:refresh_eof(Pid) method to refresh the EOF position
by reopening the backing write fd for the file.

Change-Id: I427646e0a0707a2ccaca5af8a354552dc184dfc8
Reviewed-on: http://review.couchbase.org/30989
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>