server: refactor batch construction (#24843)

* server: refactor batch construction

* wip

* wip 2

* wip 3

* wip 4

* add abort_all_slots

* handle batch full more carefully

* fix assert

* rm debug log

* small nits

* (debug) add timings

* debug: force llama_synchronize for accurate timings

* address comments

* disable DEBUG_TIMINGS

Author: ngxson