Masked email: the fork's subspace and the masked_email module (ME-1, ME-2, ME-3, ME-6a, ME-7, ME-8, ME-13, ME-14, ME-15)

Subspace _ holds the fork's own data, with its own SQL table and RocksDB
column family, and is part of backup. The masked_email module keeps each
mask's state, last mail and pending deadline beside upstream's record, an
index from address to mask with tombstones, and a per-account change log;
and generates addresses in the fork's format.

tests/src/utils/cleanup.rs

@@ -25,6 +25,7 @@ pub async fn store_destroy(store: &Store) {
         SUBSPACE_INDEXES,
         SUBSPACE_DELETED_ITEMS,
         SUBSPACE_SPAM_SAMPLES,
+        store::SUBSPACE_INBUXA, // inbuxa: masked email
         SUBSPACE_BLOB_LINK,
         SUBSPACE_LOGS,
         SUBSPACE_IN_MEMORY_COUNTER,