opt: locality optimized scan for queries with a LIMIT clause

This commit adds locality optimized scan support for queries which
place a hard limit on the number of rows returned via the LIMIT clause.
This optimization benefits tables with REGIONAL BY ROW locality by
splitting the spans accessed into a local spans set and a remote spans
set, combined via a UNION ALL operation where each branch of the UNION
ALL has the same hard limit as the original SELECT query block. If the
limit is reached by scanning just the local spans, then latency is
improved.

The optimization is not applied if the LIMIT is more than the KV batch
size of 100000 rows or if the number of spans in the scan exceeds 10000.

This commit also adds an improvement to span merging to avoid merging
local spans with remote spans in order to maximize the number of queries
that can utilize locality optimized scan.

Informs cockroachdb#64862

Release note (Performance Improvement): Queries with a LIMIT clause
applied against a single table, either explicitly written, or implicit
such as in an uncorrelated EXISTS subquery, now scan that table with
improved latency if the table is defined with LOCALITY REGIONAL BY ROW
and the number of qualified rows residing in the local region is less
than or equal to the hard limit (sum of the LIMIT clause and optional
OFFSET clause values). This optimization is only applied if the hard
limit is 100000 or less.