Add ability to use BEGIN IMMEDIATE and BEGIN EXCLUSIVE database transactions (SQLite backend)

SQLite's method of setting the transaction mode, is to add IMMEDIATE/EXCLUSIVE to the BEGIN command. Drogon currently hard-codes BEGIN.

The down side of this is potential transaction failures of the read and then write (or read and maybe write) pattern.

I suggest adding a newTransaction() variant that takes the following enum:

```
enum class TransactionType {
Default, // current behavior
Immediate, // SQLite: BEGIN IMMEDIATE
Exclusive // SQLite: BEGIN EXCLUSIVE
};
```

This would allow BEGIN IMMEDIATE and BEGIN EXCLUSIVE. Other databases could safely ignore this.