les: fix balance expiration (#22343)

* les/lespay/server: fix balance expiration and add test

* les: move client balances to a new db

* les: rename lespayDb to lesDb
# Conflicts:
#	les/client.go
#	les/clientpool.go
#	les/commons.go
#	les/server.go
#	les/vflux/server/balance_test.go
#	les/vflux/server/balance_tracker.go