Support conditional read operation

[albertwang-ibm]

We should give client the option of performing a conditional read. This should save bandwidth and reduce latency for frequently accessed resources.

From https://www.hl7.org/FHIR/DSTU2/http.html#cread:

Clients may use the If-Modified-Since, or If-None-Match HTTP header on a read request. If so, they MUST accept either a 304 Not Modified as a valid status code on the response (which means that the content is unchanged since that date) or full content (either the content has not changed, or the server does not support conditional request).

Servers can return 304 Not Modified where content is unchanged since the If-Modified-Since date-time or the If-None-Match ETag specified or they can return the full content as normal. This optimisation is relevant in reducing bandwidth for caching purposes and servers are encouraged but not required to support this. If servers don't support conditional read, they just return the full content.

https://touchstone.aegis.net/touchstone/scriptexecution?exec=201909271137469410753433&qn=/FHIR4-0-0-Advanced/Patient/01-read/fhir-api-r4-patient-read-04-conditional-found-notmodified#WAITING_OPER

[albertwang-ibm]

This could be low priority for us before we don't support this in our capabilitystatement, and touchstone knows that we don't support this by checking our capabilitystatement.

[albertwang-ibm]

verified in touchstone test, so close this issue