Refactor callers of String.toVertxPgClientPreparedSql to work for different DBs

[ShreckYe]

The formats of prepared SQLs for PostgreSQL and MySQL are different in Vert.x SQL Client. The function String.toVertxPgClientPreparedSql should not be called for MySQL. Ideally, this function should be moved to the "postgresql" module.

PostgreSQL:

SELECT * FROM users WHERE id=$1

MySQL:

SELECT * FROM users WHERE id=?

The function:

exposed-vertx-sql-client/core/src/main/kotlin/com/huanshankeji/exposedvertxsqlclient/DatabaseClient.kt

Lines 53 to 61 in cbf6692

	@ExperimentalEvscApi
	fun String.toVertxPgClientPreparedSql(): String {
	val stringBuilder = StringBuilder(length * 2)
	var i = 1
	for (c in this)
	if (c == '?') stringBuilder.append('$').append(i++)
	else stringBuilder.append(c)
	return stringBuilder.toString()
	}