MB-6657: Use 64 bits type for file offsets on Windows

Windows follows the LLP64 data model:
http://en.wikipedia.org/wiki/LLP64#64-bit_data_models

This means both the int and long int types have a size of 32 bits
regardless if it's a 32 or 64 bits Windows system.

And Windows defines the type off_t as being a signed long integer:
http://msdn.microsoft.com/en-us/library/323b6b3k.aspx

Therefore we can't use off_t on Windows if we deal with files that
can have a size of 2Gb or more.

By using off_t to represent offsets and doing arithmetic operations
on offsets of this type, such operations result in negative values
when they should produce a value larger than 2Gb. Such negative values
are then converted by os_win.c operations into unsigned 32 bits
integers, as 'Offset' members of a structure of type OVERLAPPED.
This means we can end up overwriting previous data from the file
because couchstore opens files in read/write mode (and not in append
only mode) and does "pwrite" operations with an offset stored in
the db record which is of type off_t and gets incremented after every
write operation.

Other databases and VMs use a custom 64 bits integer on Windows to
represent file offsets, rather than the type off_t.

References:

SQLite:

http://www2.sqlite.org/cgi/src/artifact/eabd00b813577d36bd66271cb08dd64ea0589dac
(function seekWinFile for example)

Python:

http://bugs.python.org/issue12517

Erlang:

https://github.com/erlang/otp/blob/2d03ab7ffde828d3b56f8b7738fd0c00d0f5b630/erts/emulator/drivers/win32/win_efile.c#L1048

Change-Id: Ib49ac055e3073480f696576cc17c791d0349b773