add doc design of tx pool

core/tx_pool.md

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+# TxPool design
+---------------
+
+
+
+## Functionalities
+1. Being a storage of coming transactions
+2. Managing all tx policies/filters (see below)
+3. Handling logic of accepting new txs and generating a new block of tx
+
+
+## Policies
+
+### PriceLimit
+This policy is to ensure that minimum gas price of a coming transaction should be enforced for acceptance into the pool.
+
+### PriceBump
+This policy is to accept a later transaction from the sender with the same nonce of an accepted transaction and drop that accepted transaction if the gas price of later transaction is (100+PriceBump)% more than the gas price of the accepted transaction.
+
+### AccountSlots
+This policy is to limit the number of executable transactions slots per account.
+
+### GlobalSlots
+This policy is to limit the number of executable transactions slots of all accounts.
+
+### AccountQueue
+This policy is to limit the number of non-executable transactions slots per account. The non-executable transactions are stored in a queue data structure. See later.
+### GlobalQueue
+This policy is to limit the number of non-executable transactions slots of all accounts. The non-executable transactions are stored in a queue data structure. See later.
+### Lifetime
+This policy is to limit the amount of time non-executable transactions are queued.
+
+### DropOldTx
+This policy is to drop all transactions that are deemed too old
+
+### DropLowBalance
+This policy is to drop all transactions that are too costly (low balance or out of gas)
+
+## Data Structure
+
+Coming up later.