Introduce analyze_sample_percentage variable

The variable controls the amount of sampling analyze table performs.

If ANALYZE table with histogram collection is too slow, one can reduce the
time taken by setting analyze_sample_percentage to a lower value of the
total number of rows.
Setting it to 0 will use a formula to compute how many rows to sample:

The number of rows collected is capped to a minimum of 50000 and
increases logarithmically with a coffecient of 4096. The coffecient is
chosen so that we expect an error of less than 3% in our estimations
according to the paper:
"Random Sampling for Histogram Construction: How much is enough?”
– Surajit Chaudhuri, Rajeev Motwani, Vivek Narasayya, ACM SIGMOD, 1998.

The drawback of sampling is that avg_frequency number is computed
imprecisely and will yeild a smaller number than the real one.

sql/sql_class.h

@@ -622,6 +622,7 @@ typedef struct system_variables
   ulong optimizer_selectivity_sampling_limit;
   ulong optimizer_use_condition_selectivity;
   ulong use_stat_tables;
+  double sample_percentage;
   ulong histogram_size;
   ulong histogram_type;
   ulong preload_buff_size;