Proposed context state improvements

[aacuevas]

ONI contexts should include an INVALID state in run_state, checked as usual, set when a call to oni_init_ctx fails. This will force the destruction of any invalid context instead of enabling the possibility of calling init twice, which could result in leaks.

Adittionally, calling oni_set_opt for ONI_OPT_BLOCKREADSIZE or ONI_OPT_BLOCKWRITESIZE when run_state == RUNNING should result in an error.