(ITS#8227) syncprov should use more threads

[hyc@openldap.org]

Full_Name: Howard Chu
Version: 2.4
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (70.87.222.79)
Submitted by: hyc


Currently a single background thread and output queue is used by syncprov to
send persist messages to all consumers. A single slow consumer can block
messages from being sent to other consumers. syncprov should be enhanced to use
a separate queue and thread per consumer.