Race Condition With Explicit Database for Transactions

# STEPS TO REPRODUCE
1. Create an in-memory database
```
val database: Database = when (type) {
is Type.Memory -> {
// https://github.com/JetBrains/Exposed/issues/726
val cfg: HikariConfig = HikariConfig().apply {
jdbcUrl = "jdbc:sqlite::memory:?foreign_keys=on"
}
val dataSource = HikariDataSource(cfg)
Database.connect(dataSource)
}
}.apply {
transactionManager.defaultIsolationLevel = java.sql.Connection.TRANSACTION_SERIALIZABLE
}.also {
transaction {
SchemaUtils.createMissingTablesAndColumns(
SubjectTableDefinition
}
}
```
2. Create a transaction with an **explicit** reference to the database
```
val fixtureSubject = SubjectFixture.newIdentifiedSubject()

transaction(database) {
SubjectTableDefinition.insertBlocking(fixtureSubject)
}
```
where
```
object SubjectTableDefinition : UUIDTable("subject") {
val sex = enumeration("sex", Sex::class)
val yearOfBirth = integer("year_of_birth")

fun insertBlocking(identifiedSubject: Identified) = insert {
it[id] = EntityID(identifiedSubject.id, SubjectTableDefinition)
it[sex] = identifiedSubject.value.sex
it[yearOfBirth] = identifiedSubject.value.yearOfBirth
}
```

# RESULTS
## Actual
With a high probability, an exception is thrown that the "subject" table does not exist.

## Expected
No exception is thrown, because the table clearly exists.

# NOTES
This is not 100% reproducible, as it might be a race condition. There seems to be a difference between implicit and explicit database references for transaction, where implicit references seem not to hit the issue. E.g. `transaction {}` versus `transaction(database) {}`.

It also seems less likely to occur if I use a file-based database, which might indicate that Hikari plays a role in this although this is not yet clear. (I'm only using Hikari as a workaround for #726.)

I'm only encountering this in my tests, not at runtime with my application. This might also be because my tests are setting up and tearing down databases several hundred times across a number of different tests, which could increase the probability of hitting race conditions.