Bump to 1.2.7

[Bitflash-sh]

Version bump for v1.2.7.

Why now: #51. Without it a node stops being reachable after its registration socket's path dies quietly — it keeps running, keeps looking healthy, and stops receiving blocks until someone restarts it. Seen in production on the bootstrap seed: 27 minutes, eight blocks behind, relay reachable the whole time.

Since v1.2.6:

• rendezvous: bound the wait for a dial, so a dead path is noticed #51 — the wait for a dial is bounded, so a dead path is noticed and the node re-registers on its own. TCP keepalive was tried first and measured not to work for this; the timeout is ours to enforce.
• mine: hash on every core instead of one #52 — mining runs on every core but one, or exactly what /genproclimit=N asks for. Implements a TODO Satoshi left in 2009, which stopped being cosmetic when the PoW became RandomX: one thread on a many-core box mines at a fraction of capacity and looks merely idle.
• wallet: export and import keys as text #49 — /dumpwallet and /importwallet move keys as plain text, for when Berkeley DB will not open the wallet at all.

Verified on both platforms before tagging: clean build, node re-registers after a silently killed path, four distinct mining threads reach the hashing loop and all report back, shutdown exits 0 with DBFlush running.