AliSQL: [Perf] Issue#24 SPLIT LOG BUFFER TO ROTATE LOG WRITE

Description:

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It included two kinds of optimization for log write:

1. Split the log buffer into the two buffer that has the same size with innodb_log_buffer_size.

   So that when log_write_up_to_lsn occured, the mtr commit can write private log into

   another global log buffer.

2. Split log_sys->mutex into two mutex.

   log_sys->mutex protect the log buffer.

   log_sys->w_mutex protect log file.

Those optimization can reduce the log write conflict and improve throughout.