Automatically test XDR type-switches

[2opremio]

In Horizon there are several places where we exhaustively go over union type tags.

e.g. 1 :

	switch operation.OperationType() {
	case xdr.OperationTypeCreateAccount:
		foo()
	case xdr.OperationTypeCreateAccount:
		bar()
	...
	}

e.g. 2 :

	switch effectType {
	case EffectTypeNames[EffectAccountCreated]:
		foo()
	case EffectTypeNames[EffectAccountCredited]:
		bar()
	...
	}

When adding new types (e.g. a new operation or effect) it's easy to miss updating some of those switches.

We should strive to create automatic unit tests which notify a case entry was missed whenever we add a new type.

[2opremio]

For operation types we can use reflection, since we have all the symbol names are automatically generated in xdr.operationTypeMap .

For effects we can go over the keys of effects.EffectTypeNames.

We should do something similar for all the history.Effect* symbols, we can probably use resourceadapter.EffectTypeNames and even compare that the length of effects.EffectTypeNames and resourceadapter.EffectTypeNames (or remove that duplication once and for all).

We should do something, as well for horizon/operations.TypeNames (check that all the existing operations are present).