fix: use a separate lookup map for reserving txs

[cmwaters]

This is an alternative to #1267 that solves the current problem with reserving transactions by using a separate lookup map as suggested by @rootulp.

[rootulp]

LGTM

[evan-forbes]

perhaps a bit excessive, but we could also add an explicit test against reading

func TestReadReserved(t *testing.T) {
	store := newStore()
	tx := types.Tx("tx1")
	store.reserve(tx.Key())

	assert.Nil(t, store.get(tx.Key()))
	assert.False(t, store.has(tx.Key()))
	assert.Len(t, store.getAllKeys(), 0)
	assert.Len(t, store.getAllTxs(), 0)
}