fixInnerObjTemplate: set inner object template (#4906)

Add `STObject` constructor to explicitly set the inner object template.
This allows certain AMM transactions to apply in the same ledger:

There is no issue if the trading fee is greater than or equal to 0.01%.
If the trading fee is less than 0.01%, then:
- After AMM create, AMM transactions must wait for one ledger to close
  (3-5 seconds).
- After one ledger is validated, all AMM transactions succeed, as
  appropriate, except for AMMVote.
- The first AMMVote which votes for a 0 trading fee in a ledger will
  succeed. Subsequent AMMVote transactions which vote for a 0 trading
  fee will wait for the next ledger (3-5 seconds). This behavior repeats
  for each ledger.

This has no effect on the ultimate correctness of AMM. This amendment
will allow the transactions described above to succeed as expected, even
if the trading fee is 0 and the transactions are applied within one
ledger (block).

src/ripple/protocol/impl/InnerObjectFormats.cpp

@@ -25,6 +25,9 @@ namespace ripple {
 
 InnerObjectFormats::InnerObjectFormats()
 {
+    // inner objects with the default fields have to be
+    // constructed with STObject::makeInnerObject()
+
     add(sfSignerEntry.jsonName.c_str(),
         sfSignerEntry.getCode(),
         {