Previously, the L1 gas price was being fetched from
a remote node and being held in memory. Now the L1
gas price is in a smart contract and the `SyncService`
will periodically update the L1 gas price in an in memory
cache to ensure that users that are sending transactions
are paying enough.

This also reads the overhead from the `OVM_GasPriceOracle`
and rejects transactions with too low of a fee.

The `tx.gasPrice` can now be variable, it no longer
will reject transactions if the gas price isn't exactly
the hardcoded number.

The cache is now updated when the sender of a transaction included
in the chain is from the current gas price oracle owner address.
This depends on the cache being initialized at startup.