common: Remove trailing _ from fields in serializable classes

According to the Googe Code Style:

  Data members in structs should be named like regular variables without
  the trailing underscores that data members in classes have.

Change-Id: I24cfb86667614dff6ab9bbf31e9396dc96011e57
common: Add class String8Bit

This commit introduces class String8Bit (std::string serializing with
8-bit length) to cope easier with legacy network messages.
Minor MooseFsVector optimisation is also performed.

Change-Id: I55456a8183f17a045e20d0d4d2d5eed8f296d7ef