* perf: reset xDomainMessageSender to default value after external call

Fixes https://github.com/ethereum-optimism/contracts/issues/309

We also set `xDomainMessageSender in the `initialize` function in L1, and in the L2 constructor.

* test: ensure that xDomainMessageSender is always 0xdead before/after calls

* chore: use parent constructor for initializing the L2 xdomain sender

* fix: require that the xDomainSender is not the default value