- Take care of posts using older public key when key is replaced.

notes: not very difficult, GetTransaction must receive a maximum block number to search the
transaction (we get this from post["height"]). another txIndex should be set to speedup lookup
(key in db includes the number of the block that changed tx so previous one can be found).
pseudocode:
   getTxIndex( key = "userX" ) => block h contains this tx;
   while( h > max_h )
        getTxIndex( "userX_h" ) => block h contains the previous tx

- Count UTF8 chars in acceptSignedPost to proper limit the 140 characters.

- Encrypt user_data (which contains all DMs)

- Test wallet encrypt to see if it still works from original bitcoin implementation and what
are the implications to our code.

- Rescan directmessages after importing a privatekey (importprivkey)

- Check libtorrent's limitation on the number of pieces (max_pieces in piece_picker.hpp = 1<<19)
Since post number is constrained by max of 288 posts per day in average, that means we have 5 years
to think about it (for the really heavy users).

- Besides increasing the maximum number of pieces, a more pressing issue to save bandwidth and
torrent download time would be to define the first piece to download/store locally. People don't
need to maintain the entire post history for everybody they follow, they could just keep the last
ones. This has to be implemented.

- Move all crypto to javascript, store only encrypted version of the privatekey (which would be
decrypted only in browser memory). getposts may obtain all DMs encrypted to browser, another
newpostmsg needs to be provided to receive posts with signature field added.

- Store a dht resource "publickey" containing not only the public key itself but also information
needed to validate it by a lightweight client. That includes: block hash, block height and partial
merkle tree inside that block. This resource propagation cannot be sent right after user
registration for obvious reasons (no block yet, other nodes wouldn't accept the signed dht put).

- Discuss and implement the acceptable level of spam per day (priorizing localization).

-