Add configurable ipc timeout

[SatpalSandhu61]

(Re-raising previously approved PR#145, after resolving merge conflicts from privacy enhancements.)

Currently, the IPC timeout, when quorum communicates with tessera, is hard-coded to 5 seconds.This can cause issues in some cases, for example when a large number of private transactions are sequentially sent to Tessera. Here is one such issue raised in Slack: https://go-quorum.slack.com/archives/C825QTQ1Z/p1600188900086900

This PR allows the timeout to be modified. The change leverages the fact that the envar PRIVATE_CONFIG can already be set to a config file instead of the actual socket file, e.g.: PRIVATE_CONFIG=qdata/dd1/tx-ipc-config.toml

The toml file should contain contents like:

socket = "tm.ipc"
workdir = "qdata/c1"
responseHeaderTimeout = 5

Note that the following timeouts can be specified:

• dialTimeout is timeout when connecting to socket (seconds), default = 1 second
• requestTimeout is timeout for the write to the socket (seconds), default = 5 seconds
• responseHeaderTimeout is timeout for reading a response from the socket (seconds), default = 5 seconds

[nmvalera]

Why not exporting the function?

Also I think NewPrivateTxManager(cfgPath string) could take the config path as an argument possibly returning an extended error using fmt.Errorf(...) so in the end, MustNewPrivateTxManager(cfgPath string) only handles the panic behavior:

func MustNewPrivateTxManager(cfgPath string) PrivateTransactionManager {
    ptm, err := NewPrivateTxManager(cfgPath string)
    if err != nil {
        panic(err)
    }
    return ptm
}

[SatpalSandhu61]

The function is only used locally, hence there's no reason to export it.
I feel it's cleaner as implemented, splitting the load config from the txn manager creation, so I prefer to leave it as is.