More fixes for blockchain corruption on OSX.

As we'd previously learned, OSX's fsync is a data eating lie.

Since 0.8.4 we're still getting some reports of disk corruption on
 OSX but now all of it looks like the block files have gotten out of
 sync with the database. It turns out that we were still using fsync()
 on the block files, so this isn't surprising.
autotools: fix a parallel build race condition spotted by the pull-tester

libleveldb.a and libmemenv.a should be able to build in parallel, but in
practice calling the leveldb makefile ends up rewriting build_config.mk. If
one target tries to build while the other is halfway through writing the
.mk, the make ends up in an undefined state.

Fix that by making one depend on the other. This also reorders the variables
to be passed by param rather than via the environment, and combines the targets
into a single rule to avoid needless duplication.
Merge pull request #3001 from theuni/fix-make-race

autotools: fix a parallel build race condition spotted by the pull-tester
Merge pull request #3000 from gmaxwell/more_osx_fsync

More fixes for blockchain corruption on OSX.
Added new config option noautofillkeypool.

One of the most annoying things about trying to maintain wallet backups is the fact that every time you unlock a wallet the key pool gets automatically refilled, thus making any existing backups of the wallet obsolete. The user now has the choice to disable automatic refilling of the keypool by adding the following line to bitcoin.conf:

noautofillkeypool=1

This effectively disables all automatic key generation, requiring the user to manually run keypoolrefill to replenish the wallet when the key pool runs low. It is recommended to call backupwallet immediately after calling keypoolrefill.

If noautofillkeypool is not set in bitcoin.conf, the app defaults to its old behavior thus ensuring full backwards compatibility.
Added ability to refill key pool from Bitcoin-Qt.
Got rid of the wallet default key since it isn't being used at all anymore and only produces bad behaviors in Bitcoin-Qt.