This patch removes client/sql_string.cc and moves sql/sql_string.cc to
sql-common so that it can be used by both client and server.

This removes code duplication. It turns out that the differences between
client/sql_string.cc and sql/sql_string.cc were mostly due to bugfixes only
applied to the latter (Bug#56492, Bug#44793, WL#1213, Bug#24293).