db_text: implement fetch and memory constraints

[lazedo]

when dealing with large db_text files, pkg_memory is not suitable for
operating the database.

implementing fetch allows modules like presence & registrar & usrloc
to query large tables without constraints on pkg_memory.

creates tmp tables in shared memory for query results

[lazedo]

this pull requests represents a changing behaviour and in principle shouldn't be in 4.4, but db_text is unusable in 4.4 with large db_text files, so i consider this as a fix for the stable branch

[miconda]

For clarification: with new features we mainly refer to new functions/parameters to configuration file or rpc commands as well as how the configuration file is interpreted. Reworking a part of code in a module is not always a new feature, to be discussed from case to case, evaluating the impact on the other modules.

I understand where the problem is with large dbtext tables + pkg memory and that this can be a fix. I am fine with merging and backporting.

[lazedo]

@miconda awesome! thank you