Fixes #485: Suggest next patient ID based on the largest number used

[gnowoel]

The suggested patient ID keeps getting incremented even if we cancelled the new patient form. This problem should be related to issue #485 and more info can be found in #485 (comment).

I try to solve the problem by saving the current sequence rather than the next available one. We start checking from the current sequence when preparing a new patient form. The value would be incremented and saved only if it has already been taken. So, no matter how many times we click the cancel button, the suggested patient ID would always be the same.

Please correct me!

cc @HospitalRun/core-maintainers

[gnowoel]

Added a commit to solve the first half of the problem mentioned in #485 (comment). That is, when saving a patient, update the sequence accordingly if the patient friendlyId is larger.

[jkleinsc]

@gnowoel thanks for the PR! Looks good to me. Changing to this approach introduces the issue that two users could concurrently get the same sequence, but I think perhaps we should address that in #572.

I also like the fact that this addresses the issue of users canceling new patient creation which was eating up sequences.

[gnowoel]

@jkleinsc Thanks! You're correct. We don't lock the sequence any more, so it's required to also address #572 to prevent duplicates.