This commit fixes to issues in `txsub` package. First, issue was a
deadlock in `Tick()` function:

```go
sys.tickMutex.Lock()
if sys.tickInProgress {
  logger.Debug("ticking in progress")
  return
}
sys.tickInProgress = true
sys.tickMutex.Unlock()
```

When two `Tick()` methods were called simultanously one of them could
`Lock()` the mutex without calling `Unlock()` it properly. This could
basically break the `txsub` system as sequence number could not be
updated.

The second issue was a problem with `App` initialization.
`App.networkPassphrase` was set to test network passphrase by default.
If `txsub` system was initialized before updating the network passphrase
using connected `stellar-core`, the system would not be able to
correctly find the transaction result in a database as calculated hash
was different than the actual hash of the transaction.