[BUG] eXist-db does not scale up with many users

[dizzzz]

Describe the bug
@line-o reported that when many users are defined in that database, the database startup takes a lot of time. With 50k users the startup is about 12 minutes.

Note

• with less than 30k users the effect is not happening
• the database performance is not affected during normal operations

Expected behavior
Even with many users the database startup should be fast.

To Reproduce

run the following query; stop database and start.

xquery version "3.1";

for $i in 10001 to 60002
return sm:create-account("user"||$i, "password"||$i, "group"||$i)

Screenshots

Running the profiler shows a clear hotspot; I think we should use a faster )Hash)map implementation here.

Context (please always complete the following information):

• OS: macOS 10.15.7
• eXist-db version: 5.2 / develop
• Java Version [ 1.8.0_262]

Additional context

• How is eXist-db installed? [e.g. JAR installer, DMG, … ] - git
• Any custom changes in e.g. conf.xml? - none