[RFE] support propagating changes from master to backup repository

situation:
Two svn-servers hosting the same repository.
a) One server hosts the master and the other the backup/slave.
b) One server is part of a private network whereas the other is
connected to the internet and serves a public repository.

task:
Keep those two servers in sync. Note that only the one server receives
commits.

current way:
1) dumping incrementals from the master repository
2) transferring the dumps to a local account on the slave repository
3) loading the incementals into the slave repository

proposed way:
svnadmin push master-url slave-url
The master sends updates to the slave.

svnadmin pull master-url slave-url
The slave requests updates from the master.

impacts:
1) feature: Easy way to synchronize slave and master repository on
different servers with different availability.

2) security: Access to only one instead of two local accounts is
required for updating the slaves.

3) security: Compromise of one server/repository has no effect on the
other server/repository.

4) protocols: Pulling should be possible without a change of existing
protocols.

5) protocols: Pushing requires transmissions of user/date-data that is
independent of the connection used for pushing.

6) db: Details(user, date, protocol infromation) should be logged for
each push/pull event in the slave repository.

Thomas Kühne