Performance: Use Sort Indexes to Improve performance

[kohsah]

When the number of documents in the system increases, there is a significant degradation of the "Recent Documents" listing. This is not evident when the system has an optimal amount of memory, however during testing by allocating smaller amounts of RAM to the JVM, this problem became evident.

We need to apply sort-indexes to the data, to improve the efficiency of pure listing queries (where data is not filtered )
(see https://exist-db.org/exist/apps/fundocs/view.html?uri=http://exist-db.org/xquery/sort&location=java:org.exist.xquery.modules.sort.SortModule )

Current TTFB for listings with over 50,000 docs is 66 seconds .

[kohsah]

Merged sort-index branch:

c700c4b

TTFB for listing queries (currently only applied to date based recency) is down to around 1 second. (vs 66 seconds earlier)

[kohsah]

@ashwinibm See https://markmail.org/message/wfmhs4h3wouem2fb , we need to switch to this : https://github.com/wolfgangmm/memsort