pg_receivexlog add synchronous mode

Hi,

This patch implements a pg_receivexlog add synchronous mode.
Now, synchronous(synchronous_commit = remote_write) is supported.
But synchronous(synchronous_commit = remote_write), if the server crashes then WAL file may not to be flushed to disk , causing data loss.

Synchronous(synchronous_commit = on) mode offers the ability to confirm WAL have been streamed in the same way as synchronous replication.
If an output is used as a different disk from the directory where the transaction log should be stored.
Prevent the loss of data due to disk failure.

the additional parameter(-m) and replicationslot specify, that its synchronous mode.
All received WAL write after, flush is executed and reply flush position.
Flush is not performed every time write, it is performed collectively like walrecever.

Regards,

--
Furuya Osamu