Add a constant for ripple epoch offset

[gregtatcam]

Issue Description

The epoch offset is currently hard-coded in multiple files as seconds(946684800) in (core/Pg.cpp, basics/chrono.h, rpc/handlers/AMMInfo.cpp) and as days(10957) (core/impl/TimeKeeper.cpp, test/jtx/impl/ManualTimeKeeper.cpp).

Epoch offset: 2000-01-01 00:00:00 UTC is 946684800s from 1970-01-01 00:00:00 UTC

Expected Result

Define a constant

[HowardHinnant]

Here's one verbose but readable way to write this constant:

#include <date/date.h>
// ...
std::chrono::seconds constexpr offset = date::sys_days{year{2000}/1/1} - date::sys_days{year{1970}/1/1};

[Bronek]

I have this in #4775

EDIT: added to basics/chrono.h