turning MaxKeepAliveRequests into a variable

For the 1.8 release of Subversion , it is now suggested to increase MaxKeepAliveRequests beyond its usual setting of 100 [1].  To allow the apache module to handle this situation this setting needs to be made into a variable.

[1]  https://subversion.apache.org/docs/release-notes/1.8.html#neon-deleted