Optimize pending transactions filter (#9026)

* rpc: return unordered transactions in pending transactions filter

* ethcore: use LruCache for nonce cache

Only clear the nonce cache when a block is retracted

* Revert "ethcore: use LruCache for nonce cache"

This reverts commit b382c19abdb9985be1724c3b8cde83906da07d68.

* Use only cached nonces when computing pending hashes.

* Give filters their own locks, so that they don't block one another.

* Fix pending transaction count if not sealing.

* Clear cache only when block is enacted.

* Fix RPC tests.

* Address review comments.