Fix constant style in txdb.h

Summary:
Make them SNAKE_CASE and constexpr .

Also bumps DEFAULT_DB_CACHE_MB to 1024 MiB.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8318