FOR CERTAIN QUERIES TO INFORMATION_SCHEMA".

The problem was that metadata locking subsystem introduced
too much overhead for queries to I_S which were processed by
opening only .FRM or .TRG files and had to scanned a lot of
tables (e.g. SELECT COUNT(*) FROM I_S.TRIGGERS was affected). 
The same effect was not observed for similar queries which 
performed full-blown table open in order to fill I_S table.

The problem stemmed from the fact that in case when I_S 
implementation opened only .FRM or .TRG file for each table 
processed it didn't release metadata lock it has acquired on 
the table after finishing its processing. As result, list
of acquired metadata locks were growing until the end of 
statement. Since acquisition of each new lock required 
search in the list of already acquired locks performance
degraded.

The same effect is not observed when I_S implementation
performs full-blown table open for each table being
processed, as in the latter cases metadata lock on the
table is released right after table processing.

This fix addressed the problem by ensuring that I_S 
implementation releases metadata lock after processing
the table in both cases of full-blown table open and in 
case when only .FRM or .TRG file is read.

mysql-test/r/information_schema.result:
  Added coverage for bug #12828477 - "MDL SUBSYSTEM CREATES BIG
  OVERHEAD FOR CERTAIN QUERIES TO INFORMATION_SCHEMA".
mysql-test/t/information_schema.test:
  Added coverage for bug #12828477 - "MDL SUBSYSTEM CREATES BIG
  OVERHEAD FOR CERTAIN QUERIES TO INFORMATION_SCHEMA".
sql/sql_show.cc:
  Changed fill_schema_table_from_frm() to release metadata lock
  it has acquired after processing the .FRM or .TRG file for
  table. 
  Without this step metadata locks acquired for each table 
  processed will be accumulated. In situation when a lot of 
  tables are processed by I_S query this will result in
  transaction with too many metadata locks. As result
  performance of acquisition of new lock will degrade.