Time and Caching fixes (#386, #478, #479).

FIXES

- Browser caching is enabled also for topics/associations which have no timestamp. 0 assumed as default (Jan 1, 1970).
- Edit conflict detection works also for topics/associations which have no timestamp.

CHANGES

Time plugin:
    - for topics/associations send to the client their creation timestamp is available in the composite value as well
      (besides the modification timestamp).
    - the time API's timestamp getter methods return 0 as default in case no timestamp is stored
      (before -1 has been returned).
    - the `Last-Modified` response header is also set for a default timestamp value (0).

Caching plugin:
    - a request's precondiction is also checked for a default timestamp value (0).

See #386.
See #478.
See #479.