Notifications from `-blocknotify=<cmd>` ought to dedupe blocks

[kseistrup]

bitcannad --help says:

-blocknotify=<cmd>     Execute command when the best block changes (%s in cmd is replaced by block hash)

However, bitcannad will gladly send several notifications about the same block. Consider the following configuration entry:

blocknotify=/usr/bin/logger -t bitcanna %s

This can result in syslog lines like these:

Oct 19 11:27:16 beatbox bitcanna[364961]: 340ce5ab9b0131858b095126d681c0743e387badd58f281cf961fcbd2586f878
Oct 19 11:27:16 beatbox bitcanna[364963]: 340ce5ab9b0131858b095126d681c0743e387badd58f281cf961fcbd2586f878
Oct 19 11:27:43 beatbox bitcanna[365051]: 416bacf11df07259eae3e1f34d57d51c419aece830d66f607f37565475969660
Oct 19 11:27:43 beatbox bitcanna[365053]: 416bacf11df07259eae3e1f34d57d51c419aece830d66f607f37565475969660
Oct 19 11:27:43 beatbox bitcanna[365055]: 416bacf11df07259eae3e1f34d57d51c419aece830d66f607f37565475969660
Oct 19 11:27:43 beatbox bitcanna[365057]: 416bacf11df07259eae3e1f34d57d51c419aece830d66f607f37565475969660
Oct 19 11:27:43 beatbox bitcanna[365059]: 416bacf11df07259eae3e1f34d57d51c419aece830d66f607f37565475969660
Oct 19 11:28:26 beatbox bitcanna[365192]: da6f9f26e1f454d3b3b70ad3e2f398360d894a0db37c66b8e87be5e8bf2e4f41
Oct 19 11:28:26 beatbox bitcanna[365194]: da6f9f26e1f454d3b3b70ad3e2f398360d894a0db37c66b8e87be5e8bf2e4f41
Oct 19 11:28:26 beatbox bitcanna[365196]: da6f9f26e1f454d3b3b70ad3e2f398360d894a0db37c66b8e87be5e8bf2e4f41
Oct 19 11:28:59 beatbox bitcanna[365309]: 235ca9e6a9c5930fe998f1d8f0aebd16cfb8189710673f337406361464fb67ee
Oct 19 11:28:59 beatbox bitcanna[365311]: 235ca9e6a9c5930fe998f1d8f0aebd16cfb8189710673f337406361464fb67ee
Oct 19 11:28:59 beatbox bitcanna[365313]: 235ca9e6a9c5930fe998f1d8f0aebd16cfb8189710673f337406361464fb67ee
Oct 19 11:28:59 beatbox bitcanna[365315]: 235ca9e6a9c5930fe998f1d8f0aebd16cfb8189710673f337406361464fb67ee
Oct 19 11:28:59 beatbox bitcanna[365317]: 235ca9e6a9c5930fe998f1d8f0aebd16cfb8189710673f337406361464fb67ee
Oct 19 11:29:17 beatbox bitcanna[365389]: deb338a36b7240998cb87e8cf38b7ee61443a07910b1e2c83517af2e5bcbcbf8

which feels very spammy.

bitcannad ought to dedupe the blackhashes so that a notification is only ever sent when the blockhash is actually different from the previous blockhash.