For very large spreadsheets, the boost json generator
(property_tree) is extremely slow and memory-inefficient.

There is little need for generic json generator, however,
since there are exactly two nodes (rows and columns)
and each is an array of size/text pairs.

The new logic uses a string with reserved capacity
to accomodate the output and generates it in one
step.

The speed improvement is orders of magnitude (hours to seconds)
for very large spreadsheets.

Reviewed-on: https://gerrit.libreoffice.org/26480Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>
Tested-by: Ashod Nakashian <ashnakash@gmail.com>
(cherry picked from commit c9d5ff91)

Change-Id: Ifaf316c270ed6e4b923ec44189a315f69e7e9b0e
Reviewed-on: https://gerrit.libreoffice.org/26485Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>