Fix mutex handling for removing minedtx requests.

Previously, RemoveMinedTxRequest was being run from a caller which
held a reader lock for the websocket request contexts.  When
RemoveMinedTxRequest tried to grab a writer lock, it would block.
This change creates a new function, removeMinedTxRequest, that does
not grab any locks, and the caller (NotifyBlockConnected) grabs a
writer lock instead of a reader lock.