[mempool] make mempool runtime required in LibraHandle
[mempool] make mempool runtime required in LibraHandle

Closes: #2302
Approved by: phoenix-antigravity

aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core