adjust MAX_TOMBSTONE_BLOCKS constant (mobilecoinfoundation#2075)

transaction/core/src/constants.rs

@@ -18,7 +18,21 @@ pub const MAX_OUTPUTS: u64 = 16;
 
 /// Maximum number of blocks in the future a transaction's tombstone block can
 /// be set to.
-pub const MAX_TOMBSTONE_BLOCKS: u64 = 100;
+///
+/// This is the limit enforced in the enclave as part of transaction
+/// validation rules. However, untrusted may decide to evict pending
+/// transactions from the queue before this point, so this is only a maximum on
+/// how long a Tx can actually be pending.
+///
+/// Note that clients are still in charge of setting the actual tombstone value.
+/// For normal transactions, clients at time of writing are defaulting to
+/// something like current block height + 100, so that they can know quickly if
+/// a Tx succeeded or failed.
+///
+/// Rationale for this number is, at a rate of 2 blocks / minute, this is 7
+/// days, which eases operations for minting agents which must perform a
+/// multi-sig.
+pub const MAX_TOMBSTONE_BLOCKS: u64 = 20160;
 
 /// The MobileCoin network will contain a fixed supply of 250 million
 /// mobilecoins (MOB).