Move the CCoins abstraction in txdb.cpp

Summary:
It is not used anywhere else than the db migration code, so it should go there.

Depends on D512

Test Plan:
Sync a node using 0.14.6 .

Recompile using that patch, check that db upgrade process occurs properly and that the node behaves properly after.

Reviewers: freetrader, CCulianu, #bitcoin_abc, zquestz, schancel

Reviewed By: #bitcoin_abc, zquestz, schancel

Subscribers: zquestz

Differential Revision: https://reviews.bitcoinabc.org/D515