improved performance in regression tests

fox deleting large columns in Fs store

fix getting all rows in Fs service

fixed deleting store in FileService

Unicode+FFFF character caused "unexpected EOF" error message

optimization: get rid of unccecessary call to DB for each shard during search

fixed performance in XLink fields in queries

wait for the cluster sinchronization after schema changes

fixed handling of empty uri parameters

Fix bug in TaskManager causing sometimes tasks to not start

support DIFFERS set operation, same as NOT EQUALS

check invalid parameters in merge options

Spider regression: querying non-existing object by id

support metric expression alias

XLink in SETS comparison query and in DCount query