syncbase java high-level: add a few more tests

Also, a few related changes:
- migrate various constants into DatabaseOptions
  (note, many of these will need to be changed as we polish things)
- add disableSyncgroupPublishing option, so that collection+syncgroup
  creation can be fast (needed since publish is currently synchronous
  and times out after 60s)
- change "userdata" cx/sg name to "userdata__" to reduce chance of
  collisions with developer-specified names
- change get() to return null on does-not-exist
- tweak notes in get{Collection,Syncgroup} about potentially returning
  null or throwing an exception
- remove unnecessary other != null check in Id.java
- add Id.toString(), for easier debugging

And a few unrelated changes:
- add /src/main/jniLibs to gitignore (build artifacts)
- add note about race in Syncgroup.updateAccessList

Change-Id: I2f20b84ebe81fc710dfbe04fca060d23f4316c9a
9 files changed