Triage crash recovery issue: sb updates while new root buffer fails

[sanebay]

Triage sigsegv in mehdi's PR

[shosseinimotlagh]

For a root node split, we are currently writing the new root node to metablk and if we failed to flush the new root buffer, we cant recover the btree. Ideally we should write to metablk, after flushing the new root dirty buffer.