Make the VM thread-safe

[vaivaswatha]

• There are global variables whose access needs to be mutexed. Here's a non-exhaustive list.
  • JSON read/writer in Utils.cpp
  • ScillaStdout in SRTL.cpp
• LLVM context and other LLVM objects accessed, as necessary.
• The object code cache manager, both at the memory level and disk level.
• Secp256k1 context has been defined to be a global variable to save time. This isn't thread-safe.

[vaivaswatha]

With #29 the following are now not needed.

• LLVM context and other LLVM objects accessed, as necessary.
• The object code cache manager, both at the memory level and disk level.

[vaivaswatha]

With 2c6e7dc, ScillaStdout in SRTL.cpp is no more a worry.