increase object size limit for form/renew RPCs

The request size for the form/renew RPCs was previously limited to 4096 bytes. This is sufficient for a single transaction, but not for contracts that require one or more parent transactions. As a result, hosts are rejecting valid contract requests. Worse, since contracts are often formed in quick succession (i.e. within the span of one block), the presence of multiple parent transaction is not uncommon. This MR therefore increases the limit to modules.TransactionSetSizeLimit, which means hosts will accept transaction sets as large as the transaction pool will allow.