fix for the missed_blocks being too big, set to zero. v3

[entropycoset]

When trying to be a witness, and loading custom genesis where initial_timestamp is far behind, there seems to be an C++ UB
in dgp.recent_slots_filled = ... << missed_blocks
in https://github.com/bitshares/bitshares-core/blob/develop/libraries/chain/db_update.cpp#L68
because we left-shift by more than 126 (or 127, 128) bits there
https://timsong-cpp.github.io/cppwp/n4140/expr.shift#1 for C++14.

In this version, we set dgp.recent_slots_filled manually to the expected value 0 instead trying to use operator<< for this corner case.

We also print error log (and once print the longer explanation of problem).

[abitmore]

The change is maybe worthwhile, but the code seems over complicated. Personally I don't want to add such logging messages in the current code base.

BTW recent_slots_filled is a 128-bit unsigned integer, See https://github.com/bitshares/bitshares-core/blob/develop/libraries/chain/include/graphene/chain/global_property_object.hpp#L96

[entropycoset]

Just silently convert it to 0, or should program show the short elog with 1 sentence?