major improvements to pull replication

1. uses caching thread pool for remote requests
2. change tracker reads response line by line
3. TDSequenceMap used to track checkoints
4. SingleThreaded access to TouchDB improved
changes to unit tests and test activity for replication changes
new test case for TDSequenceMap
integrated branch replicator fixes
testsapp compiles again but missing test.properties and local-test.properties files
Merge branch 'replicator_fixes'