rdbms: make DBMasterPos implement Serializable

rdbms: make DBMasterPos implement Serializable

ChronologyProtector uses these classes to briefly store positions
and everytime the fields change then errors can happen when old
values are unserialized and used. Use a simple two-element map
format for serialized positions. The fields are recomputed back
from the data map.

Values from before this change will issue the warning
"Erroneous data format for unserializing". To avoid that, bump
the ChronologyProtector key version. Future field changes will
not require this.

This change should be deployed on all wikis at once.

Bug: T187942
Change-Id: I71bbbc9b9d4c7e02ac02f1d8750b70bda08d4db1