When testing ETag If-Match on sobject Account, I found no matter what entity value is assigned to HTTP Header If-Match, the REST API always returns 412 code in both GET and PATCH methods. Can you explain if this is a known issue or something i did wrong?
Here is the url i used in postman: https://na35.salesforce.com/services/data/v39.0/sobjects/Account/0014100000QHDoNAAX
HTTP Headers:
Authorization:Bearer 00D41000002lPY1!ARgAQJ6GUvAHAFi2A5vLDm8hewP9njTHFaI_kQMcZQRvg4bQD2nBQIM6KaknppPNmcZP_4XZU0Qus39OQbaa_l0b45Ln3Qhs
Content-Type:application/json
If-Match:"llQbn4wyMNQw122nBsPUwPQXf0Q=--gzip"

I'm looking for a solution to build dated relationships between 2 objects, such as Account and custom object Route. It should be many-to-many relationships with start/end dates to determine if the relations are current or not. I have searched this topic online and couldn't find the right approach, such as lookup, master-detail etc. Is there a way to create a custom relationship object to link any two objects? Your feedback will be much appreciated. Thanks. 

In our existing database, the customer number serves as the link between Account and Contact objects. Can we use that field instead of ID field provided by Forces.com? Your suggestion will be much appreacited.

In our existing database, the customer number serves as the link between Account and Contact objects. Can we use that field instead of ID field provided by Forces.com? Your suggestion will be much appreacited.