Implement NEXTID chunking. Obtain IDs in chunks of size

Implement NEXTID chunking.  Obtain IDs in chunks of size
SLAPD_NEXTID_CHUNK.  Code protects NEXTID file to ensure
its equal to or greater than nextid.  Updated on close
to actual nextid.  next_id_save() could be called periodically
if desired.  Default chunk size is 32.  Define to 1 to disable
chunking.