[refactor] create client _database module

client/src/leap/soledad/client/_database/__init__.py

client/src/leap/soledad/client/_database/adbapi.py

client/src/leap/soledad/client/_database/blobs.py

client/src/leap/soledad/client/_database/dbschema.sql

client/src/leap/soledad/client/_database/pragmas.py

client/src/leap/soledad/client/_database/sqlcipher.py

client/src/leap/soledad/client/_database/sqlite.py

client/src/leap/soledad/client/api.py

...soledad/client/examples/benchmarks/measure_index_times.py

...t/examples/benchmarks/measure_index_times_custom_docid.py

client/src/leap/soledad/client/examples/use_adbapi.py

client/src/leap/soledad/client/http_target/fetch.py

common/src/leap/soledad/common/l2db/__init__.py

common/src/leap/soledad/common/l2db/remote/server_state.py

testing/test_soledad/u1db_tests/__init__.py

testing/test_soledad/u1db_tests/test_open.py

testing/test_soledad/util.py

testing/tests/blobs/test_blobs.py

testing/tests/blobs/test_local_backend.py

testing/tests/client/test_aux_methods.py