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sql: move SQL statistics to server

SQLite provides kind of statistics concerning data holding in each index
and space. It is used to help query planer build optimized plan. Such
statistics don't depend on internal index implementation (e.g. B-tree or
LSM tree) and contain information concerning tuple distribution.
This patch moves members resbonsible statistics from original SQLite
structs to Tarantool ones. To be more precise, now index's opts contains
statistics from _stat1 space (arrays of integers and their loarithm
approximation) and more sophisticated one from _stat4 (array of samples).
It also contains set of flags to turn on/off different optimizations
(such as skip-scan).

After ANALYZE command is executed, statistics are saved into _sql_stat1
and _sql_stat4 spaces (in order to persist them). However, it should also
be loaded into in-memory structs representing indexes. This patch
reworks original SQLite routine to load it directly to newly introduced
stat struct of index.

Closes #3253

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