Export constant for the maximum time offset.

This commit creates and exports a new constant, MaxTimeOffsetSeconds,
which is the maximum number of seconds a block timestamp is allowed to be
ahead of the current time.

Previously this value was hard coded into the consensus rule path, however
it is useful better to have it defined as a constant and exported so other
callers can access it.

No consensus rules have been changed with this commit.
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.