use tabletnum instead of spilitsize/mergesize in table schema

建表时，用户不需要关心splitsize/mergesize，只需要设定一个总的tablet数目，split/merge的触发由tera自身决定，对用户透明（基于size,基于负载都可以）只要保证最终tablet数目在一个用户指定的范围内即可

The issue proposes replacing user-facing splitsize/mergesize with a tablet-count range in the table schema. Start by locating the schema and table-creation entry points, then trace how split and merge thresholds are currently applied. Done requires an agreed API and behavior for automatic split/merge decisions while keeping the final tablet count within the user-specified range.

Written by the indexing model from the issue text.