Support for a larger number of tables

[flowbehappy]

Enhancement

There are many issues in TiFlash if a cluster has too many tables, i.e over 10k tables.

• Applying schema diff is slow, which slows down queries and writes. The threshold of schema apply diff/full is fixed and short #6532
• Restarting is slow Speed up TiFlash restart. #7145
• Consumes too much memory
• Don't store table schema locally by using schema.sql #7311