[docdb] Add new flag for how many servers to wait for before the transactions table

[bmatican]

We have this code in CatalogManager, which currently waits for just RF number of TS to be up, before trying to create the txn status table. However, by default, we pick a number of tablets, that depends on the number of TS alive at moment of creation, for example 8 * number of servers. Worst case, we'll just create the table based on just the minimum (default RF = 3) tablets, so 24. In a big enough clusters, with say >24 nodes, this will not be enough to guarantee that we can get one txn status tablet leader on every node!

  if (FLAGS_enable_ysql) {
    LOG_WITH_PREFIX(INFO)
        << "YSQL is enabled, will create the transaction status table when "
        << FLAGS_replication_factor << " tablet servers are online";
    master_->ts_manager()->SetTSCountCallback(FLAGS_replication_factor, [this] {
      LOG_WITH_PREFIX(INFO)
          << FLAGS_replication_factor
          << " tablet servers registered, creating the transaction status table";
      // Retry table creation until it succeedes. It might fail initially because placement UUID
      // of live replicas is set through an RPC from YugaWare, and we won't be able to calculate
      // the number of primary (non-read-replica) tablet servers until that happens.
      while (true) {
        const auto s = CreateTransactionsStatusTableIfNeeded(/* rpc */ nullptr);

We should add a new flag for this, for a user to specify how many servers to wait for upfront, before pre-creating the table. Then our platform could specify this at universe creation, as we always know how many nodes we create in a universe. There's no backwards compatibility issues, but for a sensible default, we can just default this flag to replication_factor.

cc @psudheer21 @m-iancu