* common, core, eth, les, trie: make prque generic

* les/vflux/server: fixed issues in priorityPool

* common, core, eth, les, trie: make priority also generic in prque

* les/flowcontrol: add test case for priority accumulator overflow

* les/flowcontrol: avoid priority value overflow

* common/prque: use int priority in some tests

No need to convert to int64 when we can just change the type used by the
queue.

* common/prque: remove comment about int64 range

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Co-authored-by: Zsolt Felfoldi <zsfelfoldi@gmail.com>
Co-authored-by: Felix Lange <fjl@twurst.com>