backends/glass/glass_database.h: Document parameter to internal
method
backends/chert/chert_database.cc,backends/chert/chert_database.h,
backends/chert/chert_databasereplicator.cc,backends/,
backends/glass/glass_compact.cc,backends/glass/glass_database.cc,
backends/glass/glass_databasereplicator.cc,
backends/remote/remote-database.cc,backends/remote/remote-database.h,
include/xapian/constants.h,include/xapian/database.h,
include/xapian/dbfactory.h,net/,tests/api_backend.cc: Add new flag
Xapian::DB_RETRY_LOCK which allows opening a database for writing
to wait until it can get a write lock.  (fixes#275)