Fix/issue 3

[spengrah]

This PR addresses Sherlock issue 3, which outlines a reentrancy vulnerability that allows an attacker to bypass certain protections and, for example, change the Safe's fallbackHandler.

The fix is to modify the reentrancy guard logic to enforce that HSG.checkTransaction and HSG.checkAfterExecution can only be called in that explicit order.

Together with the existing logic enforcing that HSG.checkTransaction can only be called from within Safe.execTransaction, we now enforce that

a) HSG.checkTransaction MUST be called exactly once per outer call to Safe.execTransaction, and
b) HSG.checkAfterExecution is always comparing the existing Safe state to the original/correct snapshot of the Safe state (since it MUST be called after HSG.checkTransaction and HSG.checkTransaction cannot be called after itself)

This PR updates some of the transient variable names to better describe their intent.

[gershido]

LGTM