Move specific parts of log_event.cc to log_event_client.cc and log_event_server.cc

log_event.cc has three type of the code:

• the code under MYSQL_CLIENT specific to mysqlbinlog
• the code under MYSQL_SERVER specific to mysqld
• the shared code needed by both mysqlbinlog and mysqld

Let's split log_event.cc into three parts:

• log_event_client.cc - the code specific to mysqlbinlog
• log_event_server.cc - the code specific to mysqld
• log_event.cc - the shared code

Rationale:

• It's easier to read and modify the code this way. E.g. pluggable types will need changes in both mysqlbinlog and mysqld parts.