Track down performance slowdowns with ExportWriter

Just to be clear for users glancing through this Jira : The performance slowdown is currently on an unreleased version of Solr so no versions are affected by this.

While doing some benchmarking for SOLR-12572 , I compared the export writers performance against Solr 7.4 and there seems to be some slowdowns that have been introduced. Most likely this is because of SOLR-11598

In an 1 shard 1 replica collection with 25M docs. We issue the following query 

/export?q=*:*&sort=id desc&fl=id

Solr 7.4 took 8:10 , 8:20 and 8:22 in the 3 runs that I did

Master took 10:46

Amrit's done some more benchmarking so he can fill in with some more numbers here.