[1.0.0] Paging session limits / request ID validation / misc fixes. - #263

A couple mostly misc fixes / guards:

• Adding a cap on paging sessions so it can't grow indefinitely by one client. Caps at 25. But it's configurable globally and with the normal per entry rules.
• Fixing message ID validation. My original read of the RFC was incorrect when I first implemented this long ago. It should only validate currently executing operations.
• Very minor adjustment to reject newlines in attribute use in PDO. This wasn't a security vector really, but should still be rejected.
• Fixes a read replica race condition on binds. The race condition was very very rare and discovered when I switched forwarding to occur much quicker in tests. Basically we need to re-read the current state of the entry before making a final "is it locked" decision due to sequencing. The alternative is to wrap the whole bind process in a locking transaction, which is a lot more effort for something that is very difficult to hit. I may revisit this.